Would it be possible to post a sample workbook (twbx file)?
I'm having the same exact issue. Here's a screenshot of my workaround.
As you can see, I made a three-step color in such a way that all negative values are red, all values between 0 and 50 are green (this would include all conceivable percentages) and all values greater than 50 would be black (this should include all conceivable quantities). However, if my quantities for less than 50, this workaround would not be sufficient. I'm also looking into Multiple Axis Crosstabs. I think you should as well.