Gubar

Yes, spot on, just by creating a gray image (but keeping it in RGB mode) just add a color tint to it.

Look at a spec map like a bump map, the white is the highest/brightest point or peak if you will and the darker areas are the lowest parts where lesser or no light can bounce.

As for a using in a glass Im pretty sure it would work that way, though it may need combining with a mild transparency map too so there are areas that aren't see through

Cheers
Jay