well, if you are using photoshop you could (hypothetically speaking) take your diffuse map, desaturate it, maybe tweak the contrast/brightness levels...but this only works where you have a diffuse map already in use. really depends on your application and/or what you are working on.

light color is a bump, dark is depression.

if you have an exisiting bump look at it and at the diffuse map and you'll see what i'm talking about with desaturation.

hope this helps.

Comitted to excellence through art.