We already display the value of msg.topic in the top left of the message which would appear to satisfy your requirement to be able to show two message properties within the confines of what's already there.
Thanks @knolleary. msg.topic doesn't achieve the same thing in this case because these are elements of a single message from mqtt - so they share a msg.topic. Edit - nonsense!
I can use a switch on msg.payload.device with multiple hard-coded and appropriately named debug nodes "Paint mixer", "Target colour", etc.
Thanks for the suggestion.
Actually it's not for a paint mixing machine but trying to debug interaction between individual RPi / ESP connected neopixels (Christmas is coming!).
If msg.payload is a hex colour code, the debug output includes a colour swatch, as you see in the original post.
If payload concatenates the device name and hex code, the swatch is lost, so no good for this application.
@knolleary's suggestion of changing msg.topic is (of course) the best dynamic solution
It only works if msg.payload is a string "#xxxxxx". An inject node #ff00ff works for me, but in some circumstances eg output from exec node I have to pass it through a function to lose a trailing carriage return.