According to the American Humane Society, roughly 95% of all cats are short hair cats (called domestic shorthair). This applies not just to strays but cats that have owners as well. So that fact alone is enough to be able to see why most strays are short hair cats.
Through my research, I'm reading that some people claim that long hair cats are more expensive and rare cats, so the owner is likely to take better care of them and keep them indoors. This further inflates the percentage of stray short hair cats out on the street. I couldn't find much about this from an official source, but instead just through forum posts.
As for the colors, I'm not entirely sure. The cat's fur color comes from its genetics. So it's possible that darker more earth-tone colors help the animal survive better compared to say a lighter color or white fur. Due to natural selection and survival of the fittest, the darker hair color cats will eventually out-number the lighter fur colors. That's my theory anyway. I couldn't find much data or studies to back that up.