Symbolism, Imagery, Allegory
You might have guessed from the fact that Egyptian tombs are so lavishly decorated that they have a thing for visual art, and that includes colors. And because pretty much everything in ancient Egypt is symbolic, colors have symbolism attached to them, too.
We at Shmoop kind of like the color red, but don't tell the ancient Egyptians that, because for them, red was bad. The fact that Set's evil red pyramid is, well, red might be a clue. Bast explains the symbolic system to Sadie and Carter: "As usual, modern folk have it backward. Black is the color of good soil, like the soil of the Nile. You can grow food in black soil. Therefore black is good. Red is the color of desert sand. Nothing grows in the desert. Therefore red is evil" (18.116).
In that light, it is kind of strange that Desjardins has a house in Paris with a red door, since red is "the color of chaos and destruction" (18.114). This information is one of the things that lead Sadie and Carter to suspect that Desjardins might be Set's mortal host. The take-home point is that colors aren't just for interior decorating; they might convey important information about character and alignment.
There are also some personal associations with colors. Amos, snappy dresser that he is, usually has color-coordinated outfits. On Christmas morning, this is how he looks: "His tailored suit was made of blue wool, he wore a matching fedora, and his hair was freshly braided with dark blue lapis lazuli, one of the stones Egyptians often used for jewelry. Even his glasses matched. The round lenses were tinted blue" (6.69).
We also get a clue that magical styles have some color associations. When Sadie destroys the library door in the Brooklyn mansion, the hieroglyph that appears during the spell is golden. But as Carter points out, "Dad and Amos both used blue. Why?" (7.45). We're never told exactly why, but it probably has something to do with which god you might be hosting, what your personal magical specialty is, and so on.
Hey, we never said that all the color associations made perfect sense—just that they were symbolic.