Invert
Turn every color into its negative.
Each channel is reflected about mid grey, so black and white trade places and every hue becomes the one opposite it. A partial amount moves each channel that fraction of the way towards its reflection, which is what filter: invert() in a browser does.
On an export that keeps an HDR grade the reflection is anchored at SDR white, so anything brighter than reference white inverts to black.
Throws
when amount is not a finite number.