Jonsey, as he tried to explain, you are right, the max is 255 but normally the $rgb will accept higher numbers only it does as follows, 256 is interpeted as 1, and 257 = 2, and so on, well the problem here is 255 is black and 1 is white so if u were increasing a variable to make a gradiant color change then going above 255 would mess it up because it would go straight from black to white with no blending of all the colors in between.

What this code does is makes it work back down to 1 from 255 as you increase the number past 255, so, instead of 256 being interpeted as 1 it gets interpeted as 254 so that 252 253 254 255 256 257 258 would actually be the same as doing 252 253 254 255 254 253 252 instead of 252 253 254 255 1 2 3.

Hope that made sense.