How do you say black in Latin?

For masculine it is Niger, negri, etc. For feminine it is nigra, nigrae, etc. For neuter it is nigrum, nigri, etc. Like most adjectives, "niger" always has first declension endings even when paired with nouns of second, third, fourth, or fifth declensions.

But the color black is ater.
Niger (masculine), nigra (feminine), nigrum (neuter).