See below image. I’m noticing in my datasets that when a value is 0.0, and I have a colorbar range set to [0.0, <value>] that the Use Below Range Color option is being used for values equal to or less than the minimum value, rather than simply less than.
A simple way to replicate is to create a Box source, add a Generate Surface Normals filter, and then specify to use the Above/Below range colors:
Notice how the maximum value (+1) is respected (surface is red, not yellow), but the minimum value (-1) isn’t (surface is green, not blue). Seems like even if the cause is some floating-point nonsense, rather than a <= instead of <, that it might make sense to do a tolerant comparison.