If you have a wild card on the table within a set, and you later pick up a true color needed for that set, can you return the wild card to the cards you are holding in you hand, or must you leave it on the table?
You can't take it back to your hand. From the rules:
Cards can NEVER go back to a player's Hand
However, you can move it to be part of a different set of properties on your turn:
Use Property Wildcards as substitutes for property cards of one of the colors shown on the card. You can swap these around among different sets on your turn.
So, if you have two Oranges and a Red, and are using a wild as a third Orange, you can move the wild to be a Red any time on your turn. You could do this for lots of reasons, including to increase the values of the Reds for charging rent, to avoid the Deal Breaker card, or because you drew a third true Orange and want to replace the wild with it.