The two new ride colors are actually "wild" so they don't make their own ride but you can add them to any color ride you're already building. They do have one-time penalties associated when you fill them up though--the gold ride costs 2 denari per space and the rusty one makes you take on additional dirt.
"when you fill them up with the same color"
The only changes I see to accommodate a fifth player is that the game lasts 5 rounds, not 6.
The dice for the 5th player are white, while my original copy (PL) had more "wooden" tone for them. Not that it matters, but still, they're not the same.