Answer: So that they can hide in gaudy places.
They are a bit more obvious when they visit the feeder. If you want Goldfinches in your garden then I recommend feeding sunflower hearts, they do love them.
The bird above is a male. Sometimes it is easy to tell the sexes apart. It is the red mask. On the male bird it continues behind the eye and on the female it stops short. So in the picture below the female is at the front. In practice though some male birds masks only just go beyond the eye and you can hardly see it and in some females the mask goes almost right to the end of the eye. It isn’t always easy to tell them apart.
All of these little birds make me happy.