"Secret gold" is the same as "MP100", which is the bright gold pearlescent on the pure/brushed gold lsc paints. If you paint a vehicle in "secret gold" it has the same texture as pure gold. There is an NPC-modified Surfer which spawns with "secret gold" wheel color and primary paint.
Or I have a modded insurgent painted entirely in "secret gold":
Ok in that case I don't have that available yet but since I may be purchasing the PS4 save editor very soon I will try to make one with MP100 rims and see how it turns out.