In General

Bases with "Rare" genetics are always bo. Many bases with "Special" genetics also are, however, applicator bases aren't bo since they're kinda easy to obtain. Craftable bases are so cool but not technically bo but whatever.

NCL/Raffle/Groupie/Stud

Abyssinian, Ambrosia, Asali, Bandit, Beet, Bushveld,
Celestial, Damu, Dawn, Dhahabi, Haze, Jellyfish,
Leonid, Lilac, Mushroom, Nomad, Nudar, Orchid,
Outlaw, Pearl, Pecora, Prismatic, Ripe, Ruffian,
Sahara, Scoundrel, Sepia, Sidereal, Skyward,
Stratosphere, Sunrise, Sunset, Sunspot, Ukame
Vagabond, Wine

Combo Bases

Arabica, Asiatic, Basalt, Black Rose, Cairngorm,
Celsian, Citrine, Cloudburst, Constellation, Elysian,
Ember, Ennead, Esker, Gilded, Hellebore, Hexaplex,
Hoarfrost, Madagascar, Majivu, Mistletoe,
Mudstone, Nadir, Nun, Ogdoad, Olice, Pulsar,
Qahir, Ra, Rime, Rose Gold, Serruria, Smog,
Snowflake, Soul, Sphinx, Squall, Styx,
Temporal, Witch

BO skins are NCL exclusives, tied to a base, or from event studs.
Bunting, Clouded, Echo, Eggshell Blue, Greige,
Ink, Lilac, Pale, Peach, Plum, Quail, Ruffian,
Sandstone, Welsummer

NCL/Raffle/Groupie/Stud

Aegean, Albinoid, Apricot, Banana, Celadon,
Ceramic, Chamotte, Cherimoya, Coconut,
Cordovan, Daisy, Dawn, Dove, Draconid,
Fern, Flax, Galaxy, Gazania, Gem, Glass, Glaze,
Grape, Guava, Hyacinth, Iris, Kiwifruit, Maneater,
Maroon, Mosaic, Moss, Neutron, Ocean, Pearl,
Phase, Preon, Rose, Slate, Smalti, Starshine,
Stellar, Sunglow, Terracotta, Tessera, Vicious

Breen Unique & Combos

Any hetero or sectoral eyes are combos of their components. Then there are a handful of true combos:
Aquamarine, Cosmic, Coffee, Dusk, Euphorbia,
Pine, Shamal, Spacetime, Wistful

Breed Unique are "fails" to pass a parent eye color in the same sense that PF mut is a "fail" to pass the primal mutation.
Burnish, Clay, Dunt, Eglomise, Faience, Mire,
Porcelain, Rose Ebony, Rose Quarts, Terrazzo

Shapes

Pariah, Royal, Savage, Tsavo

Colors

Algae, Amber, Anjeer, Ashen, Briar, Cairngorm,
Cameo, Citrine, Champagne, Chartreux, Cimmerian,
Cinnabar, Clay, Cocoa, Copal, Dinar, Ebony, Elysian,
Fiery, Flaxen, Flint, Fulvous, Goldenrod, Honey,
Lilac, Madagascar, Marigold, Nacarat, Orchid, Pearl,
Pistachio, Platinum, Prune, Ruffian, Qahir,
Scoundrel, Sepia, Serruria, Shedua, Slate, Soul,
Taupe, Ukame, Umber

Definitely BO

Tier 2 (raffle), Tier 4 (rosettes), and Tier 5 (hybrid/restricted) markings are always bo. The only way to get a marking with these tiers is to breed from a lion that has them already or hope and pray to the RMA gods.

Sometimes BO

Uh Oh I'm About To Be Controversial!!