About the solution
If you are using "Desaturate Alerts" in a DV or a Treemap in Combination with an Alert function you can easily see, that the the color is more desaturated, when the Value is getting smaller.
In some cases you might want to invert this for negative numbers (so the most desaturated negative values are those near by zero).
You can easily do this using a simple NEXEL Formular:
if([@a;*;*]<0;MIN( [@a;*;*;Range.Vertical] )-[@a;*;*];[@a;*;*])
As you can see for negative Values the figures are inverted (so that the smallest negative figues are the biggest negative value and vice versa)