In most cases, colour blindness is a significant anomalous perception to certain colours (ie the colours seem shifted compared to the colour seen by a normal person), or it is a significant insensitivity to those colours (ie that component of colour seems to be missing compared to the colour seen by a normal person). A total inability to see any colour at all, (ie, true colour blindness, where the person sees only in shades of grey) is quite rare. Maybe five in 100,000 people have true colour blindness.
The more normal types of colour blindness are far more common. About 8% of males are colour blind, while about 2% of females are colour blind.
You can try a simple test for the most common kind of colour blindness here.
A site that simulates colourblindness and which also tweaks images to make them more accessible to colourblind people is here.