I used to use a different colored cock feather, even dyed some myself. However, I'm with Pat on this one...I now use an indexing nock and love it. I like the simplicity of three identical feathers, plus I don't have to look at my arrow to correctly align it on the string. This is great for follow-up shots on game or when you're caught sleeping on your stool in your blind and a gobbler decides to buzz your tower!
Alternately, you could tie in an indexer in front of or behind the feathers, assuming you wrap them. I've used tiny hackle feathers from my fly tying rig.