I'm getting back into gouache after taking a long break from them. It has proven to be an excellent sketching medium. If you don't know what gouache is, they feel like watercolors and can be used transparently, but you can also use less water and get a thicker, more opaque consistency. They also dry super fast (annoyingly so if you want to work wet-in-wet), which is great for sketching. Contemporary illustrator Kai Carpenter is among the skilled few who use the medium. I hope more illustrators dabble with it given the amazing results it can produce.
Here I am using dark green and medium grey. Any two colors can be used as long as you have a light tone and a dark tone.
These sketches are drawn from the work of sculptor Sabin Howard. It was a little harder to render smooth as opposed to furry surfaces. More transparent layers and lifting with a damp brush was required. I should note that these are very small sketches. The standing statue is about 5.5 inches tall. With gouache you can work very small and get fine detail. However, working large does complicate things. For example, it is harder to get smoother transitions and even washes.
My favorite part of using gouache is the ability to work on toned paper! You just can't do that with watercolor.