Color vision deficiencies
People with color vision deficiencies see color differently than other people. Some have reduced contrast vision, some don't see colors at all, a lot of people can't differentiate between two colors.
The color palette used in libadwaita should be checked against different deficiencies, if the default palette isn't suitable for certain deficiencies there should be an alternative palette which works.
Apps which recolor the UI should be aware of kind of deficiency to make sure the recolored UI is still usable.
Apps might want to know about the kind of deficiency to accommodate their own use of color.