What would be the difference between: "Die Hemden sind nicht blau" and "Die Hemden sind kein blau"? Is the last one wrong?
As fas, as I know, You must use 'kein' to negate a noun. The last sentence negates the color, not the noun itself. That's why I think its wrong.
It says "Die Hemden" could mean shirts and undershirts/undervests (whateveer that means?) ? So I could use this word to refer to like a regular t-shirt and an undershirt/vest?
I'm not sure if this is applicable everywhere in Germany, but we use das Hemd for shirts with buttons on the front (and we have die Bluse for a blouse). An Undershirt is referred to as das Unterhemd, though it's common for undershirts for women to be called only das Hemd, too – or often das Damen-Hemd if you're shopping. We have das Polohemd for polo shirts, though the expression das Poloshirt is also used. T-shirts are usually called the same: das T-Shirt. So I would not translate Hemd with t-shirt.
Das Hemd - The shirt, Die Hemden - The shirts. Remember the article is die when the subject is plural. =)
I suggest to go ahead and visit Google translate and learn all the colors now, they are very easy to learn.