Why do clouds turn red/black?

A couple of monthes ago I noticed a lot of black clouds in an overcast and it was super dark in the day, but I don't get how clouds turn black, a few hours later the sky is dark red and black and then completely dark-red, I asked my teacher why clouds turn black and he replied with "Clouds turn black because there is heavy droplets in the cloud and since there is, the cloud blocks out light." "Why do clouds turn red?" "There are no such thing as red clouds." "What." "You must be seeing things." "No." "Then you are colour blind." I then left, I guess I am color blind and that there is no such thing as red clouds.