Article from Donna Zagotta on Gouache (Opaque Watercolor) - it's a great read. Couildn't have said it better

Opaque Watercolor

By Donna on November 11, 2009

D. Zagotta, The Mission Inn

You can’t learn techniques and then try to be a painter. Techniques are a result. Jackson Pollock
I’ve had a number of requests from readers to write about opaque watercolor, so here goes! 
10 years ago I found myself at a crossroads with my art. I was a traditional watercolorist and no longer satisfied with my work. I longed for a more creative approach, a more creative image, and a style all my own. I also wanted to be able to paint in a spontaneous, improvisational manner - to be able to respond to and work with whatever happens to present itself in my painting. So, long story short – no longer willing to settle for paintings that didn’t satisfy me, I embarked on a journey to find my authentic self and a way of working that would help me achieve my goals. Because transparent watercolor doesn’t easily allow for corrections, painting out, painting over, changing my mind, or the kind of trial and error approach I was looking for, I knew that I was going to have to change my medium or at the very least alter my approach to watercolor. 
That period of struggle, uncertainty, and experimentation led to the unconventional approach to watercolor that I use today. In my technique I combine watercolor with white gouache and apply the paint in heavy layers using very little water. Because both watercolor and white gouache are water-soluble mediums, I can layer and “melt” wet colors together on the painting to create a third color that cannot be gotten any other way.  If you would like to try opaque watercolor, here are some ideas.
The similarities and differences between transparent and opaque watercolor:
- Keep in mind that opaque watercolor looks very different from transparent watercolor. Opaque watercolor has a matte, velvet-like quality and looks much like a gouache painting.
- With transparent watercolor, colors are lightened with water, whereas with opaque watercolor, colors are lightened with white gouache. 
- Darks in opaque watercolor are produced the same way as with transparent watercolor; by painting heavily with dark-valued watercolors, adding only as much water as is necessary to move the paint around.
- With transparent watercolor, saved white paper is used for the painting’s whites. With opaque watercolor, paint is used for the painting’s whites. 
- Just as it is with transparent watercolor, in opaque watercolor the artist must be in total control of the water-pigment ratio in his brush, on his painting, and on the palette. In both transparent and opaque watercolor, too much water if often the enemy as it can easily lead to mud. 
Here are some technical tips for painting in opaque watercolor:
- Techniques: There are no “right” or “wrong” techniques. With opaque watercolor you can use transparent watercolor techniques, oil painting techniques, pastel techniques, and any other technique you can think of. You can paint thinly, or you can build up your painting with a lot of surface texture, expressive brushwork, and even impasto - the possibilities are endless and limited only by your imagination.
- Brushes: Sable brushes don’t work well with opaque painting because they hold too much water. With opaque watercolor you cannot rely on washes to fill in large shapes – you must paint them in with brushstrokes of color. Larger, “stiff-ish” brushes allow more paint to be put down. Experiment with “stiff-ish” medium and large sized rounds and flats to find the brushes that work best for you. 
- Paint: White Gouache (I use Winsor Newton’s Permanent White Designer’s Gouache) and your favorite watercolors. Keep in mind that although watercolor is a transparent medium, not every color is equally transparent. For example, the cadmium colors, Yellow Ochre, and Indian Red are highly opaque - therefore these pigments naturally have a lot of body and covering power, so I have them on my palette. On the other hand, Cobalt Blue, Rose Madder Genuine, and Aureolin Yellow are highly transparent and without much covering power, so I avoid them. There are also semi-opaque, semi-transparent, staining, and sedimentary watercolors. Each tube of watercolor has a specific characteristic and in order to work effectively with the medium of watercolor, whether you work in a transparent or opaque manner, you must know the characteristics of each pigment you choose to place on your palette.
- Palette: You’ll need a palette with deep wells. And fill those wells! You need to have a lot of pigment available to you on your palette. You will be scooping  up pigment and laying it down in individual brushstrokes. Again, with opaque watercolor you cannot rely on watery washes to fill in large shapes, and you cannot “stretch” the paint that you mix on your palette with water. 
-Paper:  You can use any type of paper, smooth or textured. I like a relatively smooth surface because I want my brushstrokes to show. I have also seen successful results in my workshops that were done on cold press and even 300 # rough paper. Experiment to find the kind of surface that works best for you.  
- Experiment! With opaque watercolor there are no limits. You can paint light colors over dark colors, you can change your mind or change the painting’s direction in process, you can try any and all ideas that occur to you right on the painting, and you can make any and all corrections confidently and without fear of losing your painting. If worse comes to worse and the painting completely goes south, you can wipe if all off, dry it, and start all over again.  How liberating!