I had originally thought that it was dependent on the difference in Martial Arts level, but after sparing with the same sim numerous times, I saw that this wasn't the case. After that, I thought it may have had something to do with mood as you did, but that didn't seem to be it either. I'm under the impression that the color of speed streaks have no real significance, other than to suggest that the sims in question are highly skilled and are moving very fast.