[EBB Sightings] House finches

I know how common this bird is, yet as I sit and watch them all day 
long at my window feeder (my computer is 5 feet from the feeder), I 
noticed something. One male has red coloring at the base of his tail 
feathers on his back, which is revealed when he moves his wings forward 
a bit.
So far none of the rest have that. I checked the Percevia site and 
there is no mention of this red on his body description.

Is this just common knowledge among bird folk?