<interactive>:1:1: Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `read' at <interactive>:1:1-4 Probable fix: add a type signature that fixes these type variable(s)
What the error message says is, it doesn't know which type to read to. read has the type Read a => String -> a, and unless you tell it what 'a' is going to be, how would it know? If you say '(read.show) x :: Color' it should work just fine.
Just to sharpen the point: read cannot know from the string it processes which type to deliver. For example, try the following at the ghci prompt: read "3" -> Yields the same error as above read "3" + 3 -> 6 read "3" + 3.3 -> 6.3 read "3.3" + 3.3 -> 6.6 read "3.3" + 3 -> Error: no parse What's happening is that the type of the expression is determined by the literal on the _right_side_ of the + sign, and not by the contents of the string on the left side. The last example is a bit tricky. ghci sees 3 on the rhs, which can be of any type in the Num class (as opposed to 3.3, which has to be of a Fractional type). If unresolved, it will default to Integer (or Int?), and try to parse "3.3" as such -- which will of course fail. -- Ariel J. Birnbaum