It's better to include the hyphen. Sometimes ambiguity can arise in this kind of phrase, such as 'a light green door'. Does this mean that the door is light green in colour or is the door green and not heavy? If you put in the hyphen, there is no ambiguity, and we know that you are referring to the colour.
Welcome to the forum . The answer to this question will vary depending on whom you ask. There is no universal answer on whether or not you need a hyphen to separate nouns or adjectives when used before participles. I believe that the hyphen would be less common in American English and more common in British English. I did a quick check however and the New York Times and New Yorker both use hyphens in this situation. It's probably more a question of style with the hyphen being the more conservative and formal option and the non-hyphen being a bit more modern (and I imagine better for Twitter with those pesky character limits that our youths deal with )
Edit: Also, as the poster above said, using (or not using) the hyphen in situations of ambiguity is important.
Welcome, man of letters. This topic has been raised many times before in this forum. The general consensus had been to advise learners of English to follow the normal convention of hyphenating compound adjectives formed from a participle when they come before the noun and not hyphenating when following the noun:
"Those are legitimate-sounding excuses."
"Those excuses are legitimate sounding ."
It's always better to have a complete sentence in your question rather than a phrase. The meaning and structure of a phrase may change with context.
Thank you very much guys. I will start a separate thread asking for recommendations for a grammar textbook and a style guide. The former I need to learn what "compound adjectives" and "participles" are as well as to answer questions like the one I just posed, and the latter I need to answer question like the one I just posed and perhaps other more tricky ones.
In phrases like legitimate-sounding, fast-selling, high-flying, etc. I'd always insert the hyphen. Especially if I had to deal with a legitimate-sounding board, which is quite different from a legitimate sounding-board!