color "bubbles" by computed category

In a scatter diagram, there are really nice visual dimensions...the y and x axis, "bubble" size, and color. I dynamically compute a comparative Percentile Performance Rating for charted points using a measure. The ratings are text (need not be but are) such as "Q1, Q2, Q3, Q4" for quartile category tags.

I'd like to color the "bubbles" by these tags. In this case, there would be 4 colors, but the best I can do is color saturation which really doesn't help much. Interestingly I can't drag the PercentileRating to the Play Axis either.

Any tips or advice here? Here's an example--I'd like to have 4 colors and have that convey a pattern. Thanks!

Tom