A thought has occurred to me. My youngest child prefers toys that are black and white, or which have high color contrast. He also likes yellow. Several of my other kids have gone through a red phase, where they really liked red toys, but not Charlie. He occasionally likes other toys that have a very light colored background (such as white) with other darker colors as the pattern. Again, this could be more about light-dark contrast than the colors.

I think there is a chance he could be colorblind, not in the sense of seeing the world in shades of gray (achromatopsia) which is very rare, but the much more common red-green color blindness. Red-green color blindness runs in my family- my dad has it. I always wondered why he wore such awful clothing combinations and why he didn't appreciate my art, and why he had us tell him when the traffic lights turned green. As an adult, it makes a lot more sense.

Suggested informal tests for color blindness in small children include presenting the child (usually a boy, since it is much more common in boys) with a collection of red or green objects, for example, a bucket of red crayons with only one or two green ones, and see if he will pick out the different colored one.

Anyway, people who are colorblind can typically see yellow, orange, black, white, and pale blue normally. This would explain why he likes black/white objects (such as his penguin and orca whales) and yellow toys (ducks) and also why, at the swimming pool, he always picks up the yellow duck first even though there's also a blue one. He likes Elmo (red), but Elmo talks, and he tends to prefer any toy which talks or makes music. Hmm.