It's not the color that is the deciding factor, it's the shape of the comm... and wear is not always easy to detect with the human eye.....Trust me it's worn...it's unavoidable given the RPM a ten turn is capable of, especially over the course of many runs as you said are on it......cut it, put new brushes,properly seat them in...and see if its not twice as fast as it is now......then put it on a dyno to find out the rpm it pulls and then re-calculate your figures.....what im getting at is you cant factor in how many rpm's your motor pulled when it was new....or for that matter trust the little printout it came with......it changes constantly