Tricolour on the vertical stab?
That's been there since Adam was ab initio The RFC & AFC both had red/white/blue (red forward) on the fin in WWI. Same in various forms for RAF/RNZAF/RCAF/RAAF. Although during WWII it was blue & white only, same as the roundels, in the Pacific.
The French had a similar pattern but blue/white/red, blue forward on the fin or rudder.
That probably doesn't answer your question ... not sure why it done in the first place.