…does this also mean that the text below (demarcated in green) also gets its respective colours from certain groups of elements?…
I'm currently not in a focussed state of mind/mood to go into much details, but…
If you colour pick a colour in the Sample Skin, and it's not defined in the xml, the only
explanation is that that colour is auto-calculated by MusicBee.
If you think about what a skin xml does, this will be obvious.( edit, not entirely true, see here: https://getmusicbee.com/forum/index.php?topic=22300.msg195851#msg195851 )
If you go years back, a lot of elements were auto-coloured using some smart algorithm.
They usually worked fine in general, but skinners were not always satisfied in how they turned out for their skins, and then asked for new and dedicated skinning elements that made it possible to define them specifically.
Pretty much all sensible requests have been fulfilled on that.
(if you check very old skin xml's you will see they have far fewer lines of 'elements' than current ones)
So if you run into an element that you can not define yourself, and are not happy about how the auto-colouring algorithm makes it look:
Create a wish for a dedicated element to define it.