Gray Squirrels
Gray backs and side
White or gray bellies
White-fringed tails
Fox Squirrels
Orange or rust backs and sides
Orange bellies
Black-fringed tails
There is some overlap in appearance.
Gray squirrels may have a hint of rust on their backs and sides.
Fox squirrels may have predominantly white bellies.
What about black squirrels?
In the Chicagoland region, any black squirrel you might see is a variety of the gray squirrel.