Re: Can't find colored jos, so...........
Good call Mary. If the goal is to "mark" the ends of the jo, then I would take an oak mop handle and use a sharpie marker on it. I might even take a hard object and slightly indent the marking so that it could be felt. The assumption has to be that all of this is to help in the learning of basic movements with the jo. The goal has to be that the jo becomes an extension of the aikidoka, and no such marks would be needed.
On the other hand - you might just like a cool weapon to swing in class, or show to friends. :-)