OK, I was kinda stumped by the shirt at first too. I even looked into the Japanese version of Spiderman and his samurai like mech, leopardon to see if there was a connection there. But the more I think about it the more I think it was their way of simply saying "...And blue". Didn't the official translation say that Yoshi-P said "Red is the main color of Spiderman's outfit and red is the theme of Stormblood." But Spiderman's outfit is red AND blue.

That along with what everyone is saying about how he gets his powers from something he can then emulate, makes me start to lean more towards a blue mage.

So in short I think Yoshi-P was trying to tell us "Yeah... Stormblood has Red Mage, but it's also going to have blue."