WZ877
Can I ask why you specifically want a RAL reference? The aircraft would have originally been painted using the British BS381-range of colours which is still in use today.
Light Aircraft Grey: BS381C-627
White: White!
Black: Matt or semi-matt black for the anti-dazzle panel.
Fuselage: Signal Red: BS381C-537
Bright Roundel Blue: BS381C-110
Roundels: Post Office Red (alternatively called Cherry Red): BS381C-538.
Regarding fluorescent paint, by 1973 this would not have been painted on but stuck on using strips of "Scotchcal" fluorescent film produced by 3M. It is still available but, as with most fluorescent colours, it fades for a past-time. If you were looking at painting it then RAL 841-GL 2005 is Fluorescent Red/Orange which is a close match.
But fluorescent markings were more associated with the earlier Light Aircraft Grey overall scheme than the later red/white/grey scheme.
Hope that helps.
Regards
Lee