Colour blindness was a visual impairment that made it impossible for one to see or distinguish colours.
The Doctor was allegedly colour blind in their first two incarnations, which gave them monochromatic vision. The Doctor remained unaware of this fact until their third incarnation, for whom the colour blindness had disappeared. (PROSE: The Day of the Doctor) In another account, the Third Doctor recalled noticing the colours of the rocks on Gallifrey in his first incarnation after meeting a hermit on the mountainside. (TV: The Time Monster)
Behind the scenes
The First and Second Doctor's alleged colour blindness is a clear in-joke directed at the fact that their respective tenures were all broadcast before the advent of colour TV.