Why is my boyfriends penis two different colours? is this common?

Some ideas: Different men have different vascular patterns. A man who uses the same tight grip on the base whenever he enjoys his body while alone is likely to have more pigment (iron, relaxed vessels) further down on the shaft. It's a non-problem. As long as it works, and there are no warts or lesions suspicious for hpv (red patches on the glans should be seen by a doctor), he's probably fine.