[glhaynes]

Colors are more complex, though: people notice that the color changes and so they understand that something else is going on (something they should know about, presumably!), but don't necessarily know what it is. It causes anxiety to have a device indicating something to the user when the user doesn't know what it is that it's trying to tell them.

Which is not to say that using color would necessarily be a bad idea in this case, but it wouldn't be entirely "free".