Try painting the darks first.

Traditional watercolor practice is to work from light to dark, which makes sense since watercolor is transparent so the dark colors cover light colors. Oil, gouache, and acrylic painters often, usually, start with the darkest areas first, then they lighten as needed. Try breaking the watercolor rule and painting the darkest colors first, then use light to dark for the rest of the painting. I find when I do that it will add the contrasts that are so effective.