I'm with Va on the 'Halcyon' subject. It was used as an aircraft naming prefix. I can't honestly remember the matching colours but there were...'Halcyon nights'(I think purple). 'Halcyon breeze', (I think green), etc etc. It certainly was never used as an RTF callsign during my time at Luton. Courtline were unusual in that they didn't use trip numbers on the RTF, they filed them in item 18 of the flightplan but actually operated under individual aircraft registrations. I was at Luton from 74-89 but Va beats me by miles and probably has a much better memory!
Spiney