punctuation - Usage of hyphen when naming colors

When is it appropriate to use a hyphen when naming colors?

For instance Blue-green has a dash but Teal blue does not.

Is there some general English rule that applies?

Answer

Generally one uses a hyphen for compound adjectives where they precede the noun or noun clause which they describe.

For example, I would write 'A blue-green sea provided the background curtain to the open-air theatre'.

But I might equally say 'My favourite colour is blue green', and 'I love being in the open air'.