Only once. That is since they are all incorrectly labelled. Let's say you pick one and it's red. Then its label has to be either "mixed" or "green". if we go with "green" then the others' labels are "mixed" and "red". Since "mixed" is not true and we already picked a red one this tube contains green jelly beans. The tube that's left contains mixed colors.
Oh wait. You can only take a single jelly-bean? Then it's trickier than I thought. You can't even be sure that it's not the mixed tube if you take 10 jelly beans. It's improbable, but possible.
So in case you can only take one, you'd have to take one from the tube labelled "mixed". Then you can be sure it's of a single-colored tube. The rest works like before.
There are two doors. One of them leads to the castle at the center of the labyrinth, and the other one leads to…
> ba ba ba bum! <
There are two guards by the doors who know which is which. You can can only ask one of them. One always tells the truth and the other always lies. How will you find out which door to go through?