Welcome. First thoughts:
• How to contact Martin Phipps, Caterham archivist:
https://f327017.invisionservice.com/profile/5695-martinphipps/
• You might be able to finds something about which Motor Show from the MotorSport archives:
https://www.motorsportmagazine.com/archive/issues/
• Do you have access to a copy of Rees' The Magnificent 7?
• I have some scanned copies of the Assembly Guide and Owner's Handbook for some Sevens before the K series. Please let me know if you'd like copies.
• Cadbury Purple (Pantone 2685C) is the leading case in English law on whether you can trademark a colour. But it only applies to "chocolate, broader chocolate range and cakes". Not Sevens. : - )
https://www.osborneclarke.com/insights/uk-high-court-provides-clarity-registration-colour-trade-marks
Jonathan