At the back of your eyes you have Cones and Rods. Cones pick up colour.
Rods pick up brightness.
There are blue cones, red cones and green cones. They pick up different wavelengths of light. Colour blind people have less numbers of particular cones than normal, so they get colours confused. Some people are more colour blind than others. This doesn't mean I see in black and white, or blue monochrome, but it does mean that I confuse red, green and brown colours, and also blue and purple.