I duuno... just seem to be able to judge too high or too low better than some... But then have been doing for a bit longer than some 'normally sighted' folk. gps distance helps too.
I very much doubt it's colour vision related. Everything's black and white at night anyway isn't it?
I think it's a learned thing - I guess us 'disabled' pilots (thats what ICAO call us) just learn to do things by better means.