In particular, for indexed images, color choice will happen within the
colormap.
The following uses are made context-aware:
* Test tool options color;
* Fill colors.
These other uses will not be context-aware:
* Padding, quick-mask and padding colors;
* Out-of-gamut colors;
* Generically generated color buttons (e.g. in filter dialogs);
* Foreground selection's mask color;
* Grid colors.
