They are just colours (although as someone who works with coloured light I’d always say black is not a colour it is an absence of colour) But done like that the rest is not required to complete the grid . If you’d put Card, Deep, Day and Sabbath in the grid instead of the answer, that would have been good although fewer overlaps …

Sorry, but they are just colours. If you had made one another word in the band it may have worked. Maybe replace l people with deep then it's the beginning of bands with colours in their names.

I don't mind the bands connection but there's more than one correct combination here. You can swap green, yellow or black for (Simply) red.

Very clever, and quite good, except for the band group. The four colours could also be the colours of characters in Cluedo (Doctor Black is the victim, Professor Plum, Colonel Mustard, Reverend Green are player characters), though that still wouldn't eliminate the problem of the being able to swap 'black','green' or 'yellow' with 'red' in the flag group