Glad that I could help you @DINGO20079527. Sorry for my slow reply, life gets in the way sometimes - I’ve not flown for months.
Anyway, I have read a lot of forum posts (on this forum and on other forums), and it seems that registrations and customising them is still quite convoluted; it seems to vary from aircraft to aircraft/designer to designer.
The clearest answer that I can suggest comes from this Avsim post, which states that an aircraft’s registration colour is set in the aircraft’s ‘aircraft.cfg’: Under each section for each livery titled [FLTSIM.#], search for a line called atc_id_color=“0x00000000”. According to this AVSIM post, the eight digits required use Microsoft ARGB colour codes.
If the above fails, then this forum post suggests that you edit the aircraft’s ‘panel.cfg’: Look for a section called [VPAINTING#] with the line ‘location = exterior’, then edit the four digits to create the web colour that you want. I don’t have the time to understand the number format for creating colours, but hopefully this points you in the right direction.