I am going to assign a texture to the cut edge of the heart, but I want to have 2 colors that define different regions.

I isolate the poly selection that will receive the colors, then place a Cube in the scene positioned and scaled to frame the area of coverage (white square). This cube acts only as a placeholder for both capturing screen grabs and then later placing your texture.

Set the Cube to not render. I like to keep it around in case I need to replace the texture and want to maintain the exact placement.

 

On a Mac I can do a marqueed screen grab (command shift 4), and I grab the area defined by the cube exactly corner to corner.
Bring the grab into PhotoShop, create a layer above it, fill it with black, set the layer opacity to 50% so you can see what you're doing, then paint away.
Set layer back to 100% and SaveAs your mask.
Drop that into Colorizer
Now back to your view in editor, and use the FitToRegion command, which gives you a marquee tool that you then set to the area defined by the cube, exactly corner to corner.
(WIP)

Beats the heck out of messing with UV coords and all that mess.

 

HTH

JeremyW