The babies are pretty small and gross looking now. You can see the yellow edge of their mouths in this picture--the "gape."
Many baby birds have a color changing mouth--it's thought to help the parents see where to put the food. Baby finches have particularly interesting mouth-interiors. Some of them kind of look like they licked a psychelic black-light painting of a clown. I couldn't find a picture of the house finch's version though.
