In a small food processor, we put a cup of sugar and about 10 drops of food coloring. A few pulses, and we had the bright yellow peep color we were looking for. The sugar is slightly wet, so you'll want to spread it out on a sheet pan to dry for about ten minutes before using.
We're looking forward to using this tip when we decorate spring cakes, and make cut-out Christmas cookies. Rather than store half a dozen different colors of sugar, we'll simply make them to order as we need them.