When I wake my iMac Retina up from sleep, it does two things: First, the brightness seems to adjust whether or not I use the automatic setting a second or two after waking the computer up. A few seconds after that, there seems to be a slight shift in the color tone, it gets warmer or more yellow. Mind you, white color still looks white, but overall the hue is a bit warmer and less blue. I haven't changed any color profiles or anything like that. Furthermore, this only seem to occur when the iMac's been in sleep mode for a at least 10-15 minutes or longer.