This is my idea. Consider extreme cases:
The front 10 is blue. 10 has been shot, and two blue ones have been completed.
Take two more, both white. 12 pieces have been taken, 2 pieces are blue and 2 pieces are white.
Take two more, still white. 14 pieces have been taken, 2 pieces in blue and 2 pieces in white.
Take two more, still white. 16 pieces have been taken, 2 pieces are blue and 2 pieces are white.
Two more shots. Well, it's finally popular. 18 pieces have been shot, 2 pieces in blue, 2 pieces in white and 2 pieces in red.
So the answer is e.