The other day a color theory student asked me, "What's the advantage of using gouache instead of watercolor, or vice versa?"
There's the obvious answer to that question: Watercolor is transparent and gouache is opaque. You use what you need based on the effect that you want.
There is also the snappy answer a child of the TV age will give: "Sometimes you feel like a nut, sometimes you don't." (Yes, it's the Mounds Bar and Almond Joy tag line, but it has so many applications in life doesn't it? All the best advertising hooks into life.)
There are many other factors that bear on the choice between these two media. Personally for me I find that watercolor requires more patience—to glaze in and layer, and wait. It's not that you don't have all that waiting for the right dryness in gouache, but if you get it "wrong" in gouache you can do some rather easy and nifty things to recover. I'll take time to do a watercolor painting, but if I'm making quick sketches it seems that gouache will be more fun for me; or alternately I want to push thicker paint around, just for the sheer fun of that process.
Perhaps one crucial factor for me in choosing between gouache and watercolor is whether or not I want all my line work to show. I draw directly with ink and sometimes I nail it (whatever it was that I was aiming for in my mind whether others see that or not), sometimes I hit something "interesting" that really needs to be studied over time so that I can learn from it, and other times I really could care less if I see those lines again. In the last case that would be a great argument for using gouache.
Sometimes, however, I've already determined I'm going to use gouache and so I have to literally say good bye to lines that I might want to save or showcase. I think this practice makes me a better person. It trains me in the process of letting go, of giving up ego, of confronting humility, of watching what was good disappear only to be replaced by something that isn't quite up to snuff. All of these (and many other interchanges I've left off this short list) are important skills for living. If painting didn't help me with life I'd probably simply stop painting.
But from this you begin to see that there isn't an easy way to answer my student's question. First, it's a question only she can answer for herself, after months (or years) of experimentation, followed by great thought about what she wants to say with her paintings.
And thought about how she wants to feel when she smooths on the paint; and the type of paper she wants to use; and the final finish she wants to achieve—to name just a few more factors.
We all have certain preferences. We all see differently. With that in mind we all have different things we want to say visually.
When I was 12 1/2-13 in first form (seventh grade) we all went off to senior school (previously we'd been at another location where prep through grade six were held). We started with a new art teacher, Mrs. Osbourne. She was wonderful. Wonderful in encouraging her students' creativity, and also wonderful in that sly way insightful teachers have of keeping a potentially disruptive student occupied. (I believe that if she could train all teachers there would be no need for Ritalin prescriptions.)
I'd been using various children's supplies purchased as needed, up to this point. Mrs. Osbourne offered real paints, with real pigments. They came out of the tube like warm butter. They also smelled wonderful.
Mrs. Osbourne of course gave instruction in how to use these paints, but the students worked independently, and my independent bent was to brush on vast strokes of lavish, thick color, enjoy the process, and not worry much about tinting and glazing. Within a short while I was chalking up quite a tab for art supplies. My mom, when she got the bill was a tad upset. (She wasn't one of those moms who asked to see what you'd done at school that day, otherwise she would have had an early heads up.) I think there must have been a teacher parent conference, because the next thing I knew Mrs. Osbourne was focusing me on an endless (and economical) supply of graphite.
And when I did get the watercolors out there was more attention placed on glazing lessons.
But the point of this story is that when left to my own devices I want paint, and a lot of it, and I want that matte and thick look, and I want to see the brush strokes, and I want…
Let's just say I have the gouache gene. And Mrs. Osbourne, knowledgeable sort that she was, made sure that I understood the 19th Century British watercolor tradition, and its use of body color. She introduced me to Ruskin (for his birds and his writing) and Hunt (for his rocks and still-life paintings with minute hatching). She recognized what I needed and supplied it—so she could go and work with the other girls without interruption.
She also had another talent, the insight to know that with improved skill I would be able to ratchet it back to whatever level of thickness of application I wanted. You don't get to that knowledge unless you go overboard for a bit.
Even if you don't have a Mrs. Osbourne in your past, a little bit of detective work will help you discover what painting genes you carry.
My student sent me images of paintings she liked. She wanted me to help her hone her palette choices so she could move towards what called to her. I was surprised by the images she sent. She was studying botanical painting (which is typically a delicate and glazed-filled endeavor; and all her color theory swatches were light glazes of color built up and up to an end result which remained fairly light in value) but all the images she sent were striking in their use of large blocks of flat, bright color. Some acrylic or oil paintings had almost a stained glass look to them because of the use of heavy black outlining. I suggested to her that some gouache might be in her future.
But here's the thing—if you use good quality gouache (I use Schmincke or M. Graham brands) you can do light washes that are more or less transparent still. So any skills you learn or use in watercolor will be useful in ways you might not realize it when you start to use gouache. There are some huge differences, of course, that will cause you to do somersaults in your brain when you plan a painting—the most obvious difference is the way you can layer light colors on top of dark colors in gouache, but can't do that in watercolor, where you have to reserve your "whites."
What it really comes down to then, is what it always comes down to, "What do I want to say today with paint? Which paint will best let me say it?"
A simple list comparing watercolor to gouache would look like this:
Watercolor is great…
• when you want to layer glazes and build up transparent color that glows from the paper up.
• when you are working on white paper (light travels through the paint and reflects back up through the paint after bouncing off the paper.
• when you want to retain the visibility of your drawing, either in pencil or in pen.
• when you want to enter your work in the Transparent Watercolor Society of America's show.
Gouache is great…
• when you want large areas of flat color (almost posterized in look, built up in a string of values).
• when you are working on toned paper (which could be either the actual color of the paper or a preprinted background—preprinted with a non-watersoluble medium that won't reactivate when you add the gouache).
• when you want to hide or obscure your drawing lines.
• when you want to push your paint around and see visible and dimensional brush strokes (not just the streaky strokes you might see from doing a flat watercolor wash badly).
• when you want to blend opaque areas of color together, by laying on a second color and bringing up the first layer through the second layer. (If you do this in watercolor it tends to look like you botched your washes and are fussy.)
• when you want to do lettering (with a brush or a pen) and need the opacity to give oomph to your lettering whether or not the lettering is over a preprinted surface.
• when you want opportunities for reworking that still allow you to come out the other end with a piece that isn't totally overworked. (Though in either medium there can be a tendency to overwork, watercolor tends to be less forgiving of going in a second time to "repair" or restate something.)
Either is great…
• when you want to work in colored pencil OVER your painting—but gouache is better because its matte texture actually cries out for the pencil strokes. (That said, many people will prefer having access to the actual paper tooth through the watercolor washes, so once again, personal taste rises up.)
• when you are in the field and want to make a quick color sketch to capture the moment or make color notes.
• when you want to make great washes of color and blend them on the page to do what they may (though the results will be different in each medium).
I think now you can begin to see that it really is this simple: Sometimes you feel like a nut, sometimes you don't.
p.s. If you want to know how to paint with gouache find a former advertising art director or advertising artist or magazine illustrator who worked in the 40s through the 60s before acrylics were available and popular. They can really push the paint around. They were trained to see shape and value. Their work reflects a bravura approach to figurative painting. These practitioners aren't going to be around forever. Tap into that repository of knowledge before it disappears.