From the mailbox:
I teach beginning Spanish and the lesson material used the phrase "¿Qué color es la calculadora?" Then later in the lesson it uses the phrase "¿De qué color es tu mochila?" A student has asked what the difference is. The lesson gives no explanation at all and I can come up with nothing that really explains it in detail, though I told her the basic difference is that the latter is asking more like "What color is it made of?" as opposed to just "What color is it?" Maybe a forum topic? (though I'd like to give this girl a more erudite answer).
First of all, I don't think your lesson material used a good example. That sentence strikes me as asking, "What color is calculator?" as if calculator is a color, or at least a question you might ask if the calculator is where both the speaker and listener can easily see it.
I'd tell the student something like this (although anyone with a better explanation is invited to comment below): The standard way to ask what color something is (as when you don't know what something looks like) is "¿De qué color es ... ?" Without the de the question usually would be understood to be asking "which color" or "what sort of color," that is, about the color itself.
So, "¿De qué color es la mochila?" would mean something like asking what color the backpack is because you don't know what it looks like. But "¿Qué color es este?" might be something you would ask if you saw something strangely colored and didn't know the name of the color, like if it were chartreuse or coral or something like that. Or if you were teaching a toddler the colors you might ask the question to get him or her to tell you the name of the color. Or you might ask something like "¿Qué color es el turquesa?" if you didn't know what turquoise looks like.
Now it may be that there's some colloquial usage that I'm not aware of where the de isn't necessary when asking what color something is. But at least in this case, it's easier for me to assume the writer just left the word off by mistake. It's easy to do, as I can attest because leaving out words is one of my most common writing mistakes.