I thought entity based lights were 0-255?
I'm not sure what the high end of brightness or color is for a light entity, but it's not restricted to 255 or less. It probably depends on the compiler. I suspect '255' is a holdover from 4-byte color days, 3 bytes of RGB and 1 byte of alpha. Also, oridinary color pickers max out at 255 (a single byte's worth of number) of brightness.
But the color numbers can be ranged up and provide brightness also. It appears that a color of 255 255 255 will add 255 to the brightness. E.g., there's no difference between a light at 255 255 128 200 and one at 510 510 256 100 (twice the color but half the brightness).