In reply to Rhi Rhi: try the car paint chip repair paint comes in a small bottle with a brush already in it the perfect size for marking gear and the paint is really hardwearing. only on metal and on a place where it is not in contact with rock, i do the top of nuts and the top of friends End of the shaft / mid axel, all you have to do to check gear is grab the clip and just look at the end of everything.
spray paint - to much work not getting it everywhere
tape takes too long to put on and comes off too easy and leaves a sticky residue.