The Nintendo Wii controllers are great but is is not easy to tell them apart. I had a small piece of blue painter´s tape on one to mark it as my secondry controller which worked well enough but was not pretty by any strech of the imagination. A few weeks ago, I used Rit dye to colorize the Wii straps. The process is simple and the results are excellent.
Mix up some dye according to the instructions on the package. I used a bit more dye solution the the package stated to get a nice dark color. Now, bring the dye to a simmer on the stove, and throw in the straps. Stir constantly; keep them moving. After about 15 minutes, take the straps and hang them to dry.
You may want to rinse out the straps after they have dried once, this should help prevent any dye coming off if they get wet. Once they have dried again, hook them to to the wiimotes and enjoy!