I have a very simple calculation on my color mark to check whether a delta calculation is positive or negative:

If ([Difference])>0 THEN 'Positive delta'
ELSEIF ([Difference])<0 THEN 'Negative delta'
END

I'm using it for a circle view, with negatives sized with absolute value.

However, I can only seem to get a single color for each, as in Blue for positive, and Red for negative. I'd like to have a red gradient for my negative values and a blue gradient for my positive values. Is this possible? If the user chooses to display values >=0 then I should only see blue gradients, rather than blue to red gradient for positive values, which seems to be the default behavior.

Any ideas? Here's a quick mock-up from superstore.