Olive has a green-yellow undertone to it and tan has a pink reddish undertone to it. If you were to take several different foundations that were in the medium/tan range you would see that while they appear similar on the shelf, when you squeeze a dollop of each onto your palm one has a yellowy appearance, one has a brownish appearance, one has a pinkish appearance and I believe there are also blue undertones in some (for people with cool toned skin) and it only becomes obvious by comparison.