If you Search the forums for something like "painted tanks" (or the likes), you'll likely come upon a thread discussing some concerns about it. If I recall right, some folks commented that other colors, and I think red was one, allow the tanks to take on more heat from the sun. Not sure of the real concern here - but I think it was discussed at length.
I've been thinking of painting my rims too. The last two trips I've thrown a wheel cover and I won't travel with them installed anymore. Seems like some color will spif-up the ugly black wheels.
Post pics if you go ahead with your idea - I'd love to see!