Look at the purple area circled by the thin, huge letter O: https://i.imgur.com/MpyG11m.png
Where did these colors come from? They’re not from ImageCache.png.
The colour of a selection is a modification of the underlying colour. The non-selected colour comes from the image cache, and the colour of the selected region is a “shifted” version of that colour. In the case of “spectral selection”, the colour values are “rotated”. The colour transform is not currently a user option. A “transformation” is used rather than absolute colours so as to support multiple track colours. Track colour options are likely to be extended further in future versions of Audacity.
What do you mean by “Multiple Track Colours” for spectrograms? Will you implement more ways to render the selection, like using a transformation picture pair where the color of each pixel in the picture B is mapped to the colour of pixel at the same position in the picture A? That would be a universal solution that handles any color with customization.
Currently Audacity supports a selection of track colours for waveform views: Audio Tracks - Audacity Manual
For spectrogram view, the choice is currently colour (one option) or grayscale (one option).
More colour options are likely to be added in the future.
That’s 5 selection colors for colored view and 2 selection colors for grayscale view. There definitely is space for 7 more slots in ImageCache.png. It may slowly+surely pollute ImageCache.png with each colour option added though.