When you edit a component (part or subassembly) in the context of an assembly, colors indicate which component is being edited. In the FeatureManager design tree, the text related to the component turns blue. In the graphics area, the component turns opaque blue, and the rest of the assembly turns transparent gray. You can change the colors used in Edit Component mode, or you can turn off the colors.
You can also change the transparency used in Edit Component mode.