I was playing with my new In Colors, and I don't have all the In Color ribbon. So, I dyed some white grosgrain ribbon to match my cardstock and ink! It occurred to me to take pictures about half way through, but you'll get the idea.
I didn't dye too much ribbon to start. So I sprayed 8 times with a water spritzer and 6 drops of reinker into a bowl. This was more than enough dye mixture. I then moved the ribbon around until it was completely covered. At this point you may want to use gloves; my fingers are still pink. Be careful to protect your workspace so you don't dye your counter. And don't use your favorite bowl; my bowl still has a light pink ring in it.
Once completely covered, I dried off the ribbon on a paper towel.
I then heat-set the color with my heat gun, being careful not to heat too closely or too long in one spot, which can damage and warp the ribbon.
The final result was ribbon that matched my ink and cardstock perfectly!! It was easy-peasy to make custom ribbon!
6/18/09 update: Another example of ribbon dyeing -