I've recently updated to Android 11 and with the new brief pop-up setting I can't seem to get a specific colour for a certain app to work.
I have set all apps to be included and in Brief pop-up settings > Edge lighting style > Colour I've set 'App Colour'
In Colour by Keyword I have added "WhatsApp" with it's own colour (purple) however when I get a notification the colour is the default green
How can an app notification colour be changed if the add Keyword doesn't work?
That is @Gudge_uk . At the moment these are the only ways of changing the colours of the notification. However, this may be changed in the future so I would suggest checking for future software updates.
I'm not really contributing to your question here, but indeed I have noticed since I am using the UI 3.0 that it is no longer possible to set a colour per app. This is really a feature I'd like to have (which should be possible because in the previous UI it was), so I hope to see it soon in a software update.