It's actually amazing that it doesn't obscure the base! Fill a layer with a color and then try all the different settings....overlay, burn, grain merge, grain extract, softlight, etc and see what it does to the image. Plus, play with the opacity. A color that looks horrible at 100% can look great at 30%.
Grain merge & grain extract are Gimp terms....sorry I don't know what it's called in Photoshop, if that's what you're using.