A couple of things to point out:
- Ancient coins are supposed to be patinated, and this patina is often dark in colour. Brass, in particular, turns black after a couple of thousand years underground. An ancient coin that looks like shiny metal, or even resembling a modern circulated coin, has probably been overcleaned.
- People that take and post pictures of coins that actually are quite dark often brighten up the image so that the details can be seen in the picture. This could well have been done to the pics in your OP.
I should also add the lower pic is a silver coin; those normally clean up much better than bronze copper or brass coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis