a simple solution for watercolor paintings (for ArtRage 3 Studio Pro)
well.. I have used a "free "plugin for this experiment (only PC version)
1) "FILE / IMPORT IMAGE FILE TO LAYER" to impor your image, and apply Thredgeholder filter for create the sketch
2) Create a new layer (or more layers) and set "Blend Mode/Tint"
3) Uses water colors on the layer (or multiple layers)
I recommend these Watercolor presets:
Delicate on Dry, Dried Strokes, Dry on Wet, Wet on Wet