Photoshop help?

[dcx_badass]

Hiya doing some work in photoshop, basically got a map of a city centre, what I'm trying to do is cut all the shops into a new layer, then cut the restaurants into a seperate layer. I can do that fine. But how would I colour all the restaurants red for example, and then all the shops blue? And be able to change the colour of them at will?
Basically colour all itms in a layer at once, regardless of the actual pattern or picture?
Thanks.