How things look to someone who is colorblind?

Not known. The only comparison is that color deficient people fail to discrimminate between various colors but no one really knows what their actual perception is. They will try to describe the world in terms of what the color normal see to stay within the majority so they will call the color of a tomato red, even if their color defect does not allow that.
Shades and Grays. Just like the old black and white film. Different colors are actually brighter/darker compared to each other. These shades let colorblind patients tell some differences, but a green and blue of the same hue will appear the same. Many phone cameras or computer photo programs can give a "black and white" effect, so play with this if you can and you'll see what they see.