Let's represent the 2:1 ratio of blue to green paint using a tape diagram.
We can divide the tape into three equal parts, with two parts representing blue paint and one part representing green paint.
Since Angelina needs a total of 6 gallons of paint, we can divide the tape into 6 equal parts, each representing 1 gallon.
Now, we can see that 2 parts of the tape (or 2 gallons) represent blue paint.
Therefore, Angelina should buy 2 gallons of blue paint.