I was thinking about enhancing the cell formatting instructions. However, I definitely wouldn't allow for hard-coded colors. Instead, I was thinking about allowing a role to be applied to a cell. Then, you can use CSS to set the color as well as any other formatting you want.
I want to emphasize that this isn't a priority right now because I have so many other pressing fixes to make. However, we can work through the design on it. Perhaps you can file an issue to allow a role to be applied to a cell (keep the implementation details vague because we need to think about the most concise and efficient way to do it).