American Goldfinches (Spinus tristis) are molting into breeding plumage at this time of year. Pictured above are three males and two females.
Below are two males (note the black caps). I'm guessing that the paler bird is still in the process of molting and will become brighter yellow. Alternatively, color saturation in goldfinches is linked to their diet. They acquire carotenoids (lutein and zeaxanthin) from seeds.
P.S. A Game 6 win today for the Black and Gold from Beantown! Go B's!