What is the difference between 红 and 丹 and 赤? They all seem to mean red. When do the Chinese use one and when do they use the other?

Seems the Second refers to red pigment and the third is red in the form of light. I'm sure someone else can clarify.

The one most used is 红. Use it to describe the colour of something. Lots of other uses/meanings too, eg 眼红 (envy).

丹 is rarely used. Can only recall seeing it meaning red in some bird names. eg. 丹顶鹤. Also used for transliteration, eg Denmark 丹麦.

赤 also uncommon, most likely to see it meaning debt in 赤字.

红 Is the actual color red. Very, very common and used when talking about something that is red in color.

丹 Very rare for use of color. Original meaning I think where about the red color pigments you find in a mine\well. Very common as sound in many words such as 丹麥 (Denmark who have a red and white flag)

赤 is a radical. Used for idea of red such as in running your finance in red(deficit) or in red cliffs.

赤 is the classical chinese way to say red. Not too sure about 丹 though.