Yellow didn't have a pair either, now that I look at it.
Guess 3:
Size matters
Size 1 = red
Size 2 = orange
Size 3 = yellow
Size 4 = green
Size 5 = blue
Size 6 = purple
When a color goes up beyond 6, it wraps to 1.
Having a red dot on the top of the circle sets the color down one level.
e.g. yellow 3 with red dot on top goes down to orange 2.
Having a red dot on the right of the circle moves the circle color down two levels.
Having a red dot on the bottom of the circle moves the color down three levels.
Having a red dot on the left of the circle moves the color down four levels.
Having an orange dot on the left of the circle moves the color up two levels.
Having an orange dot on the top of the circle moves the color up four levels.
Anyways, the white circle has two red dots, one on the right, and one on the top. This should subtract 3 levels from the circle, giving you YELLOW.
IF this is wrong, I am done with this puzzle, I have to go to class...