9514 1404 393
Answer:
blue
Step-by-step explanation:
If two red marbles are removed, 1 red is returned. The number of reds is reduced by 1, and the number of blues is unchanged.
If two blue marbles are removed, 1 red is returned. The number of reds is increased by 1, and the number of blues is decreased by 2.
If one of each is removed, one blue is returned. The number of reds is reduced by 1, and the number of blues is unchanged.
So, at each step, the number of blue marbles is unchanged or reduced by 2. That is, it only changes by an even number. The number of blues is initially odd, so can never reach zero.
The last marble in the bag is blue.