When I read code, I find it really useful to see whether a method is real, or just an extension.
To do so, enable Color identifiers in ReSharper > Options > Code Inspection > Settings. Be aware, that this sets the ReSharper default colors in the first place.
Then define ReSharper Extension Method Identifier in Options > Environment > Fonts and Colors to your favorite color. I like to have them in Purple.
Have fun!
This is indeed a good idea. I hope VS2010 will allow this by default. I gave value types a different color already a long time ago, because they behave different than reference types. I spotted many bugs because of this highlighting.