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. But this all depends on the intensity of the colour and the brightness of the light conditions at the time. For instance, I will always be able to tell that a bright green crayon is green, if I'm standing outside in bright sunlight. But if I'm standing under a 40-watt bulb then I might think it's a brown crayon. And if it was a green object which wasn't as colourful as a crayon, I could even think it was red.