Let's say you have the red and green columns still available Let's say a quest requires a blue or red column.
For you to be able to get said quest you need to have at least one of those two colours available. You qualify, as you have the red column still available.
However, after getting the quest you don't have to flip the red column to unavailable, you just have to flip over a column. Therefore, in this case you can pick to either flip the red or the green column over to unavailable.