I'm not sure weather this question has to do with "Pronunciation and Accent", or "Grammar and Vocabulary", perhaps it's got a bit of both to it.
I just wonder what exactly the letter "й" does in such adjectives as "красный". Does it represent a sound? Apparently, it does not. The way I figure, "красны" would be pronounced exactly the same way. So is it just a matter of aesthetics? [I have to admit that красный looks more beautiful than красны, although it's perhaps because my eyes are more used to the former.]
Or perhaps the "й" is there to emphasize that "красный" is an adjective, and not, say, the plural of the imaginary noun "красн"?
Whatever the case, I suppose that while "й" is clearly pronounced in "сделай", it is not in "красный", "любимый", "хороший", etc. Am I wrong?