Color vs. intensity of return is calibrated up to 100 miles out in radars produced since about 1982. It may be less for some models radar; it certainly is for planes with smaller than airline standard 29 inch antenna plate. This is known as STC, Sensitivity Time Control, range. Beyond that, the radar kind of guesses at actual intensity. The farther out a return is detected, the more likely it will display red. Within the STC range, the color/return is calibrated.
Increasing the gain from CAL is effective only within STC range, as the receiver gain is already at max for returns greater than 100 miles.
What has been the experience with the newer Collins and Honeywell radars with automatic tilt and gain?
GB