Yes, glazes do come in different colors; but they are really sheer. I mainly use them to tone my hair---add warm tones or cool tones. Glazes (at least the ones I use) are demi permanents and wash out in about 4 weeks on my hair.
Yes, I use them to warm up my hair. I use an ash blonde permanent color b/c lightning my hair reveals warmth. If I used a golden blond on my hair, it would read red and not blonde. However, the ash blonds are sometimes a bit too ashy. I add some warmth using a strawberry colored glaze. The glaze makes my blonde look reddish in the sun, but overall, it just appears to be a golden blonde (you don't see the red tones b/c it's so clear). Then again, I like the clear glazes, too, because I love the shine.
For medium brown hair, I'd use a brown glaze bc it will add shine and should refresh the color so it's not so ashy. I'd try something like this
John Frieda Brilliant Brunette Luminous Color Glaze Ulta.com - Cosmetics, Fragrance, Salon and Beauty Gifts
I use glazes after I color and in between coloring, too. It depends on my roots.