Well I wouldn't wear a brown leather jacket at the same time as a black leather jacket
But in seriousness I think the black/brown thing depends on the specific items, and about formality. In a suit in a formal situation, I'd never advise wearing brown shoes with black trousers. But if it's black jeans with brown boots it can look ok. Try it on and see what you think is the general rule. Brown jacket with black jeans, maybe not. Tends to work best if it's subtle, such as shoes rather than complete block difference of black bottom half brown top half.
As for whether you should buy a leather jacket in a different colour, go for it. I'm a big proponent of having items in different colours in order to match different things. Normally I only say do that with things like t-shirts (which are fairly inexpensive) or cheap leather strap watches for example. Leather jackets are pricey for decent ones, but if you've got the funds then I think it's a good idea.