It is possible to user negative color values in Mapserver. This is not documented. The most common way is to use -1 as the red value which makes a layer transparent. However some other effects can also be achieved. I documented some of those in my Mapserver Easter Egg findings: https://lists.osgeo.org/pipermail/mapserver-users/2018-March/080547.html
Those are also documented in the GIST: https://gist.github.com/LarsSchy/df91553f1fda02196ba7881a129b4c64
This issue was rasied lately by a question on the dev-list by Seth: https://lists.osgeo.org/pipermail/mapserver-dev/2020-July/016246.html