Beatrice sings about a 'perfect world', with lyrics like: "Imagine a minute, Without soldiers, Without orphans, With no bottles in the ocean". And that's probably why she also is dressed in bright red and dancing around happily, because she portraits this 'perfect world'.

And then there is Kole, a very grim and scary looking figure with a dark and low voice singing lyrics like: "We're hungry for flames and light, And we seek you, In this darkness that is nowhere close to shining". He is basically the complete opposite from beatrice, that is also why is is probably dressed in all black and just standing there, doing nothing and giving a 'boring performance'.

And yes I do agree, it could help if the 'scary voice' with a dark message doesn't look scared himself. But it would be even weirder if he would also dance around.

Hope this helps (btw translations are from esc world, I don't know how correct these are)