Make cake. While stripes are baking, use icing colors to tint 2 tablespoons of remaining batter green, making a darker shade. Gently fold tinted batter into remaining batter. Carefully pour green batter over baked stripes. Use spatula to evenly distribute batter in pan.
Bake cake 10 to 12 minutes. Cool in pan 10 minutes, then loosen the sides of cake from pan edge. Turn out of pan onto a second piece of parchment paper. Gently peel off parchment from pan and turn cake back over. Starting at short end, roll cake so pattern shows on outside and cool completely.