I'm color blind, and I could care less what color something is; can't see it anyway. My wife has been going nuts over what "color" to paint our family room for weeks: 50 shades of white! She keeps trying to get me involved, and I keep ducking.
If your friend is anything like me, he could care less whether his armguard is green or brown. He probably wouldn't see the humor in it, nor would he be offended. He would probably just appreciate the arm guard for the craftsmanship you have no doubt put into it.