The first (abandoned) version is D6532, if anyone is interested.

Adding @Campbell Barton (campbellbarton) as reviewer since he coded toolbar icons, and @Clément Foucault (fclem) since he commented on previous implementation.

Hope the other reviewers are ok with me removing them from this - if not, feel free to say so.

I believe all colors here are in sRGB space.

Please convert to linear colors before doing any operations and convert back to sRGB for storage.

Also I would like to see if just decomposing to hsv and inverting the Value component is better or just the same.

It seems faster and more correct to change the colors in BKE_icon_geom_rasterize before rasterization, instead of after.

Then, maybe not to invert colors every time at re-rasterization, but once at loading .dat-files and store two sets of colors.

Or maybe even better to store two prepared sets of colors in .dat files.
In this case it can be manually prepared materials as William originally suggested in the T64177.

@Brecht Van Lommel (brecht) @Campbell Barton (campbellbarton) ?

To me that seems fine, it's probably a more effective way to get good looking results. But @Campbell Barton (campbellbarton) and @William Reynish (billreynish) know this system better than me.

In D6649#156953, @Clément Foucault (fclem) wrote:

I believe all colors here are in sRGB space. Please convert to linear colors before doing any operations and convert back to sRGB for storage.
Also I would like to see if just decomposing to hsv and inverting the Value component is better or just the same.

Using HSL now, gives the same result. I tried to use linear colors and HSV, for some reason it doesn't produce good results at all.

In D6649#156965, @Brecht Van Lommel (brecht) wrote:

It seems faster and more correct to change the colors in BKE_icon_geom_rasterize before rasterization, instead of after.

Now it changes colors before rasterization.