Hi, Noah, thanks for writing us with your question.
I'm not sure which book you were reading, and I have never heard of such languages myself. To be honest, I kind of doubt there really are such languages. Have you ever heard about how Eskimos have lots of words for "snow"? Well, at least a lot of people think that's just wrong. It's a myth. In this case, I find it hard to imagine that the people speaking any language wouldn't find it useful to have words for more colors than the ones you mention. And if it's useful, then they will introduce such words.
But let's suppose that there are languages like that and ask what we should say about them, if so. Both options you mention seem possible: that they have words for "black", "white", and "colored", and that they have words for "black", "white", and "red". In the latter case, then, as you say, they would have no word for the color of the sky. But they could still describe it, if they had a word meaning "same color". They could say the sky was the same color as the lake, maybe, and that the grass was the same color as the leaves on the tree.
If they couldn't describe it, and felt they needed to do so, then, as I kind of said before, I think they would probably make up a word for it. Languages grow that way all the time. They are alive, changing every day to fit the needs of the people who use them.
In fact, you might even have made up new words yourself! We all do it. Mostly, we make up names for things, but sometimes we make up names for new kinds of things, even. One of my cats used to make lots of different kinds of meows depending upon what it was he wanted. So we made up words to describe his meows. We needed a word, and there wasn't one, so we created it.
Isn't language cool?