For example, Esper refers to decks made up of White, Blue and Black cards.
What are the names of the other colour combinations? In particular, how do we refer to a Red-White-Black deck?
For example, Esper refers to decks made up of White, Blue and Black cards.
What are the names of the other colour combinations? In particular, how do we refer to a Red-White-Black deck?
In the most common environments, decks with a single color are not competitive in tournaments (due to the fact that you're settling for the top ten cohesive cards in a color instead of two sets of the top five in two colors. The difference in power level between the top 1-5 and the top 6-10 can be massive and game deciding. The other colors also make up for each others weaknesses). But they do exist, especially in older formats where consistency is required to win in a couple of turns and where you have such a large pool of powerful cards to pick from, and are then referred to as "Mono X". For completeness sake, mono color tribes have been included here even though they are unlikely to be labeled as such.
They are usually referred with the names of the guilds of Ravnica. Since the guilds were released (and especially now that we've returned) the color pairs have become common names for duo-color decks. Both allied and enemy color pairs are common to see, although allied more so (due to higher deck cohesion).
The first of these are the shards of Alara. They jive well because they give two sets of allied colors each. The less commonly used are the wedges, which involve two enemy color combinations: they're derived from the dragons of Planar Chaos, the volver cycle from Apocalypse, five clans from Khans of Tarkir, or more recently the triomes of Ikoria. Having only one allied color pair (the two enemies of a single color will be allied) limits deck cohesion, making their use infrequent, although this obviously varies as the meta changes.
Note that if one of the colours is not particularly present in the deck, for example there's only a single card of a given color, you may see a two color name with a third listed as a "splash."
Informal usages:
In addition, it's especially common for red + blue + green and black + blue + green to be called by their abbreviations — "RUG" and "BUG" — because these are names that are easy to remember and pronounce, even though those are not the proper color orders.
Most decks do not have four full colors. As with three color enemies, if they reach this many colors, it's a shard with a splash of another color. So you're more likely to see something like "American splash black" instead of "Yore", and lately simply "four color" is rising in popularity.
Names for four-color identities come from one of two sources:
The four colour identities' names are:
Occasionally, some informal usages of four color decks, especially when it involves a typical three color deck with a splash of a fourth, will be to include some kind of pun or other indicator to an established name. For example, taking a Jund (Red, Green, and Black) deck and adding Blue would be more likely to make it something like "Wet Jund" than it would be Glint-Eye or Chaos. Or if only splashing Blue for a single card might make it "Moist Jund", that is Wet Jund that is slightly less wet than Wet Jund. Similar puns can be made for Dark Jeskai (Blue, Red, and White adding Black) and I could envision something along those lines for a splash like Dim Jeskai (Blue, Red, and White splashing Black) although admittedly I've never seen it in the wild.
Decks with all five colors usually revolve around a single combo that they hope to pull off. It takes a lot of mana fixing and a massive amount of playtesting to get a reliable five color deck. For this reason, you don't often find them in tournaments. You find them often in Commander (giving the player access to every card ever, greatly increasing the power level of the deck) and in skill challenges where a player just tries to come up with a crazy deck idea to see if he can make it work. Obviously there's only one five color deck, it uses all five:
Sometimes, four color decks will also be called Rainbow just because they have so many colors.
Sometimes, particularly in formats with a very large card pool, you'll see colorless decks as well. The most common name for these is a reference to the old card frame for artifact cards:
Mono brown (not to be confused with BrownTown which is a draft deck leaning heavily on minotaurs)
Some newer colorless decks have come to be referred to as "Diamond" decks in reference to the diamond-like new symbol for colorless mana
As noted by others, the 'aligned' color triplets - that is, those that consist of a color and its two siblings to either side on the usual Black-blUe-White-Green-Red color wheel - have nicknames (but not formal names!) based on their 'shards' in the Shards of Alara block: Esper (Ubw), Bant (Wug), Naya (Gwr), Jund (Rgb), and Grixis (Bru).
As for the non-aligned color triplets (which of necessity are one color and its two enemies), the 'dragon' scheme that Lee Abraham mentions based on the names of the dragons that were in Planar Chaos (Intet for Urg, Oros for Wrb, Vorosh for Gub, Numot for Rwu and Teneb for Bgw) is one that I've seen; the other (and slightly more common, at least in the playgroups I play with) is based on the 'shards' that were in Apocalypse, the original 'enemy colors' set - there's a consistent name there that's used on both enchantments and creatures that care about the enemy-color combinations. That set is:
In addition to the Ravnica Guilds listed in corsiKa's answer, Strixhaven adds the following (each of which represents one of the colleges):
WUB: Esper
UBR: Grixis
WUG: Bant
GRW: Naya
RBG: Jund
The other colour combos (two allied colours, plus their joint enemy) have not yet been "named" by another Shards of Alara-esque block, but I'm sure it will happen one day...
Other answers have addressed the 3 color combinations (aka Allied colors correspond to the Shards of Alara and Enemy colors are much less clear) but the two color combinations correspond to the guilds of Ravnica:
WU - Azorius
WB - Orzhov
WR - Boros
WG - Selesnya
UB - Dimir
UR - Izzet
UG - Simic
BR - Rakdos
BG - Golgari
RG - Gruul
I've seen some people call the other color combos by the names of the elder dragons.
For instance RWB - Oros after the card Oros, The Avenger. Other times I've seen them called based on a national flag that contains their colors. For instance RWU is regularly called American.
However the moment there are no "official" names for the wedge colored decks.
Blue white Azorious, Blue black Dimir, Blue green Simic, Blue red Izzet, White black Orzhov, White green Selesnya, White red Boros, Black green Golgari, Black red Rakdos, Green red Gruul.
I don't believe there is an answer to black-red-white. Esper refers to one of the Shards of Alara, of which there are five. From a post on TappedOut and MTGSlavation:
Bant - primary white, secondary blue/green
Esper - primary blue, secondary white/black
Grixis - primary black, secondary blue/red
Jund - primary red, secondary black/green
Naya - primary green, secondary red/white