You can unlock this as early as DNA Sequence 7. When you begin DNA Sequence 4, you are in Florence again. While you are there, make sure you stop by an Art Merchant (located by the icon) and purchase a piece of art. It doesn’t matter which painting you buy.
Then later in DNA Sequence 7, when you are finally in Venice, again, locate an Art Merchant and purchase a piece of art. There is at least one located in the San Polo district you are currently in. If there are none visible, synchronize a few View Points until one appears. The achievement will unlock shortly after.