I’ve always wondered why some dirt such as clay is white-ish or red, but most dirt is brown. Is it something to do with carbon, something like that?
My WAG is organic material, dead and alive.
Yeah, that’s pretty much it. Dirt with a high organic content is brown or even black, whereas dirt with a high mineral content is a different color.
More generally, as any artist will tell you, if you mix a lot of different colored “stuff” together, generally you’ll get browns or blacks. The fewer color components in something, the more likely it is to be a non-brown shade.
I suspect that clays and the like are simpler in structure, or have a larger percentage of their makeup being a single colored substance–whereas dirt is a mixture of more things. I’ll admit this is a guess; it could just as likely be that dirt is “simple”, but it’s primary constituent is something brown.