Um.. I think you're using a half transparent brush, and when going over a spot you colored it gets messy. If you use a program with layers, try shading with solid black/white on a new layer and make it like 50% transparent. Otherwise, color in a solid brush with lighter color(ie, lightblue on the dress). Adding a purple hue to shadow, and orange hue to light will make it look colorful (and those colors go well with the picture).
Remember that stronger light casts darker shadow, so either have less light or add shadows.