Those purple areas are very highly saturated. filmic, turned on by default if you use that workflow, will try to preserve colours in highlights, reducing brightness.
You can use sigmoid, which rolls off to white; you can tell filmic to roll off to white (set look / highlights saturation mix to 0); you can use filmic v6 or v5 (options / color science) with options / preserve chrominance set to no or luminance Y.
When filmic did not try so hard to preserve colour, people were complaining that it was producing flat highlights.
I think what’s happening is it first applies the tone curve to the brightness, and then multiplies the RGB channels with a value derived from the original colour (RGB ratios) of the pixel.
Look at what happened to a ‘purple’ (clipped) sun here, and the explanation given by filmic’s developer, Aurélien: Magenta highlights vs raw clipping indicator vs filmic white level - #60 by anon41087856.
1 Like