as far as the walls go, i would go with just the grey and blue, imo it's cleaner, and keep in mind black paint is going to make spaces seem smaller. As for the tiles, the gray tile look particularly difficult to clean, can they come like the black and blue ones? if so I would go with that as the center. if not, have you played with other ideas other than a perimeter look. perhaps mostly black tiling with a 2 tile wide band (4 tiles wide total) of blue and gray going up the length of the garage on both left and right. or what about a checkered pattern