Cassin’s Kingbird

Here’s one i know u wont have trouble with. but for those of you up to the challenge, i’d like u to detail the differences between this bird seen at mills pond this morning, and others of the “solitary Vireo complex”. (actually I think this is 2 individuals, same species).

Here’s a question: does anyone know the answer without checking field guide? (I only know cause I learned it just now. I saw it): What color is a juvenile cardinal’s bill this time of year?