People who are colour blind cannot see colours as they actually are. They might match a purple top with lime green pants because they can't see the colours and don't know the colours will "clash" according to other people.
There are different degrees of colour blindness - I can't tell the difference between colours that are similar such as cream, ecru, light beige, etc. They all seem the same to me even though they aren't. I have very mild colour blindness which has never really caused me a problem; but, some people have it to a much greater degree. I know some people who had to have their spouse help them chose their clothes for the day because they couldn't tell the difference between colours.