Black : Very rare and very bad potential level. Pokémon with black potential levels up excruciatingly slowly and will never be able to exceed level 49 unless it increases it's potential. Note that in addition to laving a level cap of 49, Pokémon with this level of potential level up excruciatingly slowly, though this can be mitigated somewhat by drawing nearer to Red potential.
Red : A much more common potential level then Black Potential, this has a level cap at level 50. The cap might only be one higher then the cap of Black-rank potential, but 50 is an important milestone level where a Pokémon enters elite rank, so there is a huge difference between a level 49 and a level 50 Pokémon. Levelling up to 49 isn't so hard for a Red Potential Pokémon given time, effort, and proper nutrition, they crossing the invisible barrier into an Elite Pokémon is very hard. It is possible, though.
Orange : A Pokémon with this potential level is capable of reaching level 74 at maximum. It can enter elite tier much easier then a Red-tier Pokémon, but cannot enter the Champion Tier by reaching Level 75. from level 50 to level 74, an Orange Potential Pokémon levels up excruciatingly slowly. This can be mitigated somewhat be approaching Yellow Potential.
Yellow : A Pokémon with this potential level is capable of reaching level 75 at maximum. The cap might only be one higher then the cap of Orange-rank potential, but 75 is an important milestone level where a Pokémon enters Champion rank, so there is a huge difference between a level 74 and a level 75 Pokémon. While the barrier between a common Pokémon and an Elite Pokémon can be crossed relatively easily, a Yellow Potential Pokémon has a very hard time passing through the carrier into Champion Tier.
Green : A Pokémon with this potential level is able to enter Champion Tier quite a bit easier and is capable of reaching level 89 at maximum. Most gym leaders will have their Pokémon at least at Green Tier potential due to taking good care of their Pokémon and training them very well.
Blue : Blue-rank potential is relatively rare potential level in the real world, The maximum level of a Pokémon with this level of Potential is 90, which is the start of Epic rank. Most Pokémon trained by gym leaders, elite 4, or champions would be at this level.
Purple : Purple-rank potential is an extremely rare potential level in the real world. The maximum level of a Pokémon with this level of Potential is 99, which is the Peek of Epic rank. Very few Pokémon at this level exist in the world naturally.
White : The maximum level of a Pokémon with this level of Potential is 100, which is Demi-Legend rank. Aside from Legendary and Mythical Pokémon, typically you'll never see a Demi-Legendary Stage Pokémon. Obviously, Death's cheat system makes this possible.
Bronze : Bronze rank is the beginning of what could be called the realm of Legendary and Mythical Pokémon. The following Pokémon are Bronze Rank if fully matured : Regice, Registeel, Regirock, Regieleki Regidrago. Azelf, Mesprit, Uxie, Heatran, Cobalion, Terrakion, Virizion, Tornadus, Thundurus, Landorus, Kubfu, Urshifu, Glastrier, Spectrier, Calyrex, Manaphy, Phione (Only the eggs of a Manaphy), Shaymin, Meloetta, Diancie, Magearna, Marshadow, Zeraora, Meltan, Melmetal. Bronze rank Pokémon can step into 'Legendary levels beyond level 100, with a level cap of 115. Pokémon at Bronze Rank can sometimes have one move not normally available to their species.
Silver : Silver rank potential has a level cap of 135, with 116 to 135 making up the mid-stage of Legendary Realm. The following Pokémon fall into this grade when fully matured : Articuno, Zapdos, Moltres, Lugia, Ho-oh, Entei, Raikou, Suicune, Latias, Latios, Groudon, Kyogre, Cresselia, Darkrai, Regigigas, Keldeo, Reshiram, Zekrom, Kyurem, Cosmog, Cosmoem, Solgaleo, Lunala, Zacian, Zamazenta, Victini, Genesect, Hoopa, Volcanion, Zarude. Pokémon at Silver Rank can have two moves and one ability not normally available to their species.
Gold : Pokémon with gold-ranked potential have a level cap of 150, with 150 being the absolute peak of Legendary Realm levels. The following Pokémon possess Gold-rank potential when fully matured : Dialga, Palkia, Giratina, Xernias, Yveltal, Zygarde, Rayquaza Mew, Mewtwo, Celebi, Jirachi, Type: Null, Silvally. Pokémon at Gold Rank can have four moves and two abilities not normally available to their species.
Platinum : Pokémon with Platinum-rank potential can enter mythical levels from 151 to 200, or 300 in the case of Arceus. The only known Pokémon at Platinum Tier are Arceus, Necrozma, and Eternatus. Pokémon at Platinum Rank can have eight moves and three abilities not normally available to their species. Yes, even when Arceus creates an avatar to appear in the Pokémon World it is a level 300 existence, so it's best not to piss him off.