The Booster Pack Rainbow refers to the cycle of background colors on Booster Pack sleeves followed by each series of OCG and TCG Booster Packs. This cycle was introduced in the OCG with Series 2, where it consisted of nine colors: green, yellow, red, orange, blue, purple, black, white, and brown. The brown was dropped after Series 3, and was not used in the TCG. The rainbow was introduced to the TCG beginning with Series 4, marking the beginning of the OCG and TCG receiving the same packs and thus following the same pattern. Other apparent patterns are: