Cool, nice idea. The more I think about how to visualise additional information in existing symbols, the more I like altering existing elements of these symbols (instead of adding new ones).

But in this case, colouring in not needed (especially as the issues specific colours carry in different cultural contexts have been recently pointed out).
"Graying out" the inner circle, making it transparent, making it "hollow" (i.e. a small circle) or omitting it completely (Edit: or letting it blink, cough, cough) might indicate various GPS states without the use of colours.