Handling dwindling supplies in Splendor
Yes you can still reserve without being able to get a gold (some disadvantage: you get the card, but not the gold)
Yes you can still take gem tokens when there are fewer than three colors left (obviously you might get fewer than normal).


Combining these two leads to a useful money-starvation strategy (esp. in 3-player games) if there is some critical color needed for either the nobles or the middle/top row cards. Reserve up to three of the critical color-generating cards (from any rows) and leave the other player with junk in the first row and excessively expensive junk in the middle row. When you run of gold, the other players are then forced to either buy junk, pick up useless color coins, reserve without getting gold, or even gamble by reserving a random (face-down) card.

Even better if you time things so that when you do purchase the cards you've reserved and return useful coins to the supply, they are near their 10-coin limit so can't (net) pick up many coins in that turn, letting you try to get your useful money back before they can get it :) and continue the money-starvation for even more rounds (you can't do that with 4+ players, someone will pick up the useful money).

In a 3-player game, if two of you collude you can mess up the third player.

In one extreme case of a game I played, 10 of the 12 face-up cards ended up being black-producing, which was totally useless for nobles and top row.