Typically, rule of thumb is to match a neutral color (white, gray, black) with a color, and vice versa. Although complimentary colors work well too.

With your red bike, white, black, or gray would look classy. Blue and Yellow would look fine too. Green would make it look "Christmassy". Orange is probably not a good color either.

With your Gray bike, just about any color you want will work, although a different value color would work well. White or black for a monochrome look would work well, or red, orange or yellow. Blues and greens might not go well. It might fade into the frame color. Or look like mould.

When all is said and done, get the one you want most. That'll please you best. Remember that there's no accounting for taste. Nor should there ever be.

Enjoy!
Chris