A lot of the stuff I’m saying about ties can probably be applied across the board — maybe. I have more grey trousers in my closet than any other color, but I have them in a variety of shades, weights and fabrics. This makes it easier.
For colored trousers, like red chinos, I try to make sure that I pair it with something simple up top — like a navy blazer, blue shirt and navy tie. You want to balance the overwhelming element against the more sober ones. Same thing goes with plaid or check pants, I’d say.
I generally try to avoid matching grey trousers to a grey jacket, even if they’re different shades. It just looks odd to me, like you couldn’t find your suit pants. In this case, I’d go with navy trousers or chinos. Or denim. Or khaki or brown pants. Or a lot of other colors, which will work because grey’s neutral.
Same goes for navy trousers/chinos with a navy jacket. I avoid that altogether.
This is why I (mostly) buy only navy/blue jackets and grey/charcoal trousers right now.