Medieval knights and colors, that's not clowns, that's historical accuracy for a medieval context. Perhaps the game goes a bit further, but then it's a tropical color, not clowns, Medieval tropical knights, that sound cool.
That's not historic accuracy because they only had their overcoats colored.
Plate-armor meanwhile, was not always accompanied by any overcoat and was just the plain metallic color of iron/steel.
The armor in Divinity games has always been fantasy-type armor, asides from in the first game where they opted for realistic armor.
I agree with the OP that some of the armor in Original Sin looks like clown armor.