The big reason many folks in ImagineFX switch back and forth is for specific toolsets that makes things easier.

Example: Start your drawing, then color washes, and painting the full image in Painter. You're done, right? No.

Save that image (TIF, PSD, dealer's choice, really) and bring it into Photoshop (PS). In PS you can do overall color adjustments, masked adjustment layers to hit large areas but not the whole thing (or the whole thing - change what you want to change), big changes, small changes, etc., adjustting tint, tone, color, and contrast.

This is what graphicnovel was getting at as far as "finishing". You can adjust individual colors or color sets - if you wanted your yellow brighter, the reds richer, the blues darker, etc. If the scales of the dragon were too low contrast you can punch them up REALLY fast with complete control - get the idea?

I hope this was helpful - enjoy painting!

-Lew