Red is rumored to attract the birds, but wouldn't it be just as effective to wrap a piece of bright red paper around the liquid jar? In any case it doesn't sound like anyone here has a real controlled experiment. ("I use red and I get lots of birds, so..." doesn't prove or disprove anything.) Snopes is reasonable as always: http://www.snopes.com/critters/wild/reddye.asp
Commercial feed suppliers are also necessarily interested in their customers believing they're doing something more special than repackaging and marketing some very expensive colored sugar water. So, neither the fact that they use red nor the fact that they exist nor the fact that what they sell seems to work is in any way an argument not to do it yourself.
