In colour blindness, it may be that a person has more rods than cones, it it may be that the cones are not fully functional - depends on the individual and the specific form of colour blindness. There are a couple of ways. First, the ability to judge visual magnitudes of celstial objects is greatly affected by how our eye's respond to colour. Ther stronger the colour is in an object being observed - whether it is a comet or a globular or a nebula, the birghter it appears. The human brain is still the most advanced image processing system in the world that we know of.
Strong colour cause the brain to interpret more birghtness.
Each colour reflects different amounts of light. How most people judge contrast in a picture or scene is based on how much light is being reflected by different colours. Depending on the picture in question, if the scene were converted to a grayscale image, much of the contrast would disappear. For someone who is colourblind, the different colours do not as easily affect judgement of contrast or brightness. The perception of contrast is based more on a grayscale.