I used to think that square or checked shirts were really cool, thats back in the days. But not anymore even though I still have a couple, from them days.
Nowadays, plain, bright colored shirts appear cooler to me than anything. And by plain I mean without any decorations, lines, color mixtures etc. Bright colors for shirts appear sexy to me for instance if the accompanying dress/pants is of a dull color and the tie/other top is also different (and preferably dull and or decorated). They also look perfect with a suit, jeans, etc and make someone appear as presentable as they can get. I don’t know if the opposite can be true or cool. Shirts can really tell much about yourself. Are they ironed, worn-out, recycled.
And of course, that multicolored, decorated shirt works for the holiday or a sunny outing. I bet it would also look cool with a suit if it had those gigantic colors that fall all the way to your shoulders.