Is there a rule about a sportcoat being darker than pants?

Generally speaking, I prefer an odd jacket to be darker than its accompanying trou, the exception being a white dinner jacket. Darker pants can work, but it's tougher to successfully pull off.

I don't feel that this applies to sweaters or outerwear.

not true for camel sportcoat/blazer or topcoat/overcoat

There is no rule about an odd jacket's needing to be darker than the trousers accompanying it. Its's just that such a combination is far and away the most commonly seen, as most men don't own a lot of light colored jackets in tne first place. So they default to the dark jacket / light pants combo with which they are most familiar and comfortable.

I have a light grey plaid sportcoat that I wear often with navy or charcoal colored trousers. The only real rule is that there needs to be a distinct contrast between the colors.

Light sport coat - darker, contrasting pants

Dark sport coat - lighter, contrasting pants

but beware of textures or patterns that fight.

I don't even have a camel sportcoat I prefer dark colored coats as I find it easier to match with garments I already have.