I always made wireframes as low-res as possible to encourage the client to focus on structures and interactions rather than colors, fonts, and copy. So I stuck to greyscale. The closer to their corporate colors (and fonts, icons, logos), the more they focused on those things.
In my experience, color is the responsibility of the Visual Design team, not the UX Design team. So if clients asked about that I'd say, "Colors haven't been determined yet. First, we need to finalize this stuff."
Also, sometimes we sent wireframe documents to the client and never knew if they would be printing the wireframes on a black and white printer.