Red is a source of food.  Hummingbirds know there is probably good food in those red feeders and red flowers.  But hummingbirds are attracted to any source of good nectar.  They will frequent flowers of all colors.

Hummingbirds see color differently than we do  They have a denser concentration of cones in their retina and they have heightened color sensitivity to the red to yellow range.  In other words, they see red better than blue or green.

But in the end, hummingbirds will go where they have learned that there is food for them.