The Core Booster rainbow refers to the cycle of background colors followed by each series of OCG and TCG Core Boosters. 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.
Starting with Series 9, each series corresponds with the 3-year run of the anime series (with Series 9 corresponding to the Yu-Gi-Oh! ARC-V anime and Series 10 to VRAINS), instead of the previous timespan of 2 years. As a result, Series 9 had twelve main booster releases instead of only eight; the first eight followed the established rainbow pattern, but the last four were respectively darker shades of blue, green, red, and yellow. The rainbow pattern changed again slightly in Series 10, with the swapping of the black and white colors in the cycle to coincide with the reintroduction in chronological order of pre-Link mechanics in the second year of VRAINS: Ritual (blue), Fusion (purple), Synchro (white), and Xyz (black). Series 11 returned white and black to the Series 9 order.
So far, every Green set has had the word "Duelist" in its name since Series 4 (the Series 2 and 3 boosters instead had "Ruler" in their names), and has either introduced a new gameplay dynamic, or elevated a previous mechanic (Magic Ruler introduced Quick-Play Spell Cards, The New Ruler introduced the first archetype, Soul of the Duelist introduced "LV" monsters, Power of the Duelist introduced fast-paced Fusion Summon strategies with "Future Fusion" and "Overload Fusion", The Duelist Genesis introduced Synchro Monsters, Duelist Revolution augmented Synchro Monsters by introducing Synchro Fusion Monsters like "Dragon Knight Draco-Equiste", Return of the Duelist introduced an attribute-based series of boosters—this one being an EARTH-based booster pack, Duelist Alliance introduced Pendulum Monsters, Code of the Duelist introduced Link Monsters, Rise of the Duelist introduced a paradigm shift of moving new anime cards to the Yu-Gi-Oh! Rush Duel game and using their space for OCG-original support of old anime themes, and Duelist Nexus introduced a new Illusion Type).
Likewise, a trend has been seen with the Black and White sets. Like their name, the Black sets introduce predominantly DARK monsters that alter the metagame at the time of that set's release (Shadow of Infinity released the Sacred Beasts and "Demise, King of Armageddon", Phantom Darkness released "Dark Armed Dragon", Absolute Powerforce released "Battle Fader", Order of Chaos released the "Inzektor" archetype, and Legacy of the Valiant released "Gravekeeper's" support). Conversely, the White sets introduce predominantly LIGHT monsters designed to counter what was introduced in the prior Black set (Enemy of Justice released numerous monsters used in Counter Fairy Decks, Light of Destruction released "Honest" and the "Lightsworn" archetype, The Shining Darkness released "Herald of Perfection", Galactic Overlord introduced the Lightray series, and Primal Origin introduced the "Artifact" archetype).
Other apparent patterns are:
- Every Blue pack features a Dragon (or Dragon-like) monster as the cover card (with the exception of Judgment of the Light and Cybernetic Horizon) that was owned by a major character in the anime.
- Purple packs feature upgraded versions of past monsters (with the exception of Photon Shockwave, though its cover card shares many similarities with "Blue-Eyes White Dragon").
- Orange packs feature the ace cards of the non-main characters, with the exception of Extreme Victory.
- Every cover card on the Yellow packs after Rise of Destiny feature the ace monsters of several main characters of the series, of which all of them are either upgraded versions or counterparts of monsters that each respective character also owns and uses.
- Red packs feature a Dragon or red-colored monsters, with the exception of Flaming Eternity and Extreme Force.
