I can't tell you how many times a student has told me their dog ate their watercolor painting. Yes there's a legitimate reason for the painting that disappeared and there is a reason why Buster thought it looked delicious.
Watercolor paint is often made using sugar or corn syrup. Our furry friends can smell that and will mistake your painting for delicious treat. It's especially enticing when you're using darker more concentrated colors while painting.
You don't usually need to worry about Buster getting sick from eating watercolors. You can even make your own using ingredients from your kitchen. Here's a great tutorial for making watercolors at home: https://happyhooligans.ca/homemade-watercolour-paints/
Obviously, you will want to keep your paints and paintings out of reach so Buster doesn't make that mistake again. Now that you know why it's easy to understand why he thought it would make a nice treat.