Actually using darker colors isn't that great at all. Your right it creates a very dirty and dark effect. I recently had an interesting conversation with a fellow artist and realized that I need to cut down on amount of dark colors Im using and move on to more saturated style. My point being, that your doing just fine with the pallet you have right here. :) You could indeed add more darker colors, but most of your pictures have a very unified and unique style, and I really like that, and wouldn't change it if I was in your place, quite the opposite I would expand on it. You seem to have a great grasp on creating a watercolor effect and those colors suits it very well! Furthermore I think this style would suit marvelously with RPG Maker! Exceptionally with the DS style! Some nice pastel portraits would do nicely~
I really love what you have here so far, I adore the 2sd and 3th picture the most~ Such a great composition there <3 I'm hoping to see more form you in the future :D
and purple... purple is life~