I would say it is a matter of preference. By definition, there is no correct answer, because we're looking at slang, not "proper" English. Personally, I use -ish rather than ish, but everyone will have their own opinion.

The rule is very simple: check a dictionary. If it is a recognized standard word, it will be in the dictionary and you spell it without a hyphen. Some examples: reddish, yellowish, greenish, bluish, darkish, nightmarish, kittenish, brackish, coltish, slowish, wolfish, sheepish, Moorish, Turkish, impish. If you are manufacturing it yourself by adding the suffolk "-ish" to a word, and it's not in the dictionary, then use a hyphen. If it's a word we all need, then maybe you will be lucky and people will start to use it and someday it will be added to the dictionary.