It's not a stupid question at all!
Solid color is a recessive gene so if two solid color cats mate, then all the kittens will be solid color.
As for the actual color, that depends. Gray is a "dilute" color which is recessive, so two gray cats will have all gray kittens. But black is a "full" color, which is dominant. So two black cats could have all black kittens, or they could have 75% black kittens and 25% gray kittens, depending on the genes each parent has.