Nope, there is no reason for this… you can make custom PCBs any color you want. Gray seams to be a neutral background color and easier to identify other colors, parts, traces, etc. (gray has no color). I have made some green ones and blue ones, what ever you prefer… Some of the green PCBs you see may be breadboard images or early Fritzing PCBs. Green has been the standard color of PCBs for many years and is best suited to symbolize a PCB.
If you want to make your custom PCB the color “chartreuse”… I can live with that… although the traces may blend in a little…