The sentence that uses correct hyphenation is the one that reads as follows: "The reddish-purple wall was ready to be repainted". Two or more words that come before a noun, modify it and act as a single idea should be hyphenated. These words are called compound adjectives. In this case, reddish-purple is a single idea because both words refer to one colour in particular: It is neither purple nor red, but purple which comes close to red.
