Riviera is a tricky one. Recommended interior colours changed at least once, same as Baikal and the high impact colours. The ECE Colours&Trim cards Sep. 73 & Feb. 74 both combine Riviera with Light Grey, for all E9 models cloth (Stoff) S0010 or vinyl (Kunstleder) K0008. USA models should be the same; some months ago a Riviera Bavaria was on offer, also with the common Light Grey vinyl.
The grey cloth yellows like hell but don't mind because it is unavailable anyway. Vinyl will be difficult, too, you need the plain and the perforated material. Moreover, grey was only introduced in late 1973.
Good news: from Sep. 73 on change of interior colour was not an option at extra cost anymore. The first owner of your 3.0 CS A might have chosen Sandbeige, Marineblau or Schwarz for the same price. Dark blue or black cloth materials were remanufactured at some time, the others not.
Last exit leather. Six colours, all available for every exterior colour. Schwarz L9090, Marineblau L4141, Beige L0303, Englisch Sattelleder L0707, Weiß L8181, Veloursrot L2727. Englisch Sattelleder = English Saddle = rich medium brown. BMW used very good leather, with regular care you can pass the seats to your children. Factory quality material today probably is not cheap. Bad news: leather interiors had matching vinyl colours for doorpanels and so on, only black and dark blue are identical to cloth and vinyl interiors. As above, you'll need the plain and the perforated material. And the doorhandles. And the carpets.