Color slots refer to a technique that evaluates how much of a deck's mana base is required to support a particular color on a given turn. The technique is best explained by examples:
For now, let us consider a deck using a basic curve, as referred to in "mana curve", and only basic lands. Let us start with "60 cards, play first". In these decks, we have a starting point of 24 lands. The second turn slot requires at least 12 lands that produce that color for a 90% probability to cast, and a third turn slot requires 11 lands - conveniently adding up to just under 24 lands. A 12/12 manabase would thus be perfectly functional for a two-color deck, assuming there are no 1-drops that need to be played on the first turn.
If this deck wanted to use double-colored mana costs then it would have to dedicate 20 sources to support the cost on turn two, 17 to support on turn 3, or 15 to support it on turn 4. For a two-color deck with MM and 1N costs, this would require a total of 32 sources, which necessitates the usage of at least eight dual lands to function properly, and with only basics means a 57% probability to cast a MM spell on the second turn. A deck with MM and NN costs would need 40 total sources and 16 dual lands, which ranges from difficult to impossible depending on the format. For triple cost or above this gets even more restrictive. Triple cost on turn three would require 22 land, or turn four would require 18. For both of these, a manabase using only basics can only support one color, as the remaining lands will have almost no consistency in supporting any cards on the curve. A deck seeking to cast a MMM card and a 1N card would need 22 and 12 sources respectively for a total of 34 sources and effectively three sets of duals. These do not even consider the fact of the downsides these lands may have, most commonly entering tapped - a non-trivial percentage of the time the curve will be thrown off because the land will not generate the right mana the right turn, and compromises will need to be made. Hence, building manabases requires more attention than is usually given.
"Reverse Mana Syndrome"
Some cards can be so powerful in a deck that players can actually "reverse" the normal way that the colors are represented with lands. Some spells have an incredible impact if played on early and on curve, but occasionally these will only be eight or fewer spells; this results in the mana skewing itself far more towards the minority than normal. Cards in this category are high-impact double or triple colored cards (such as Gideon, Ally of Zendikar, which caused this during the KTK-BFZ era), or one-mana spells (usually mana dorks).
Decks rarely exhibit this syndrome, largely because of the propensity of dual lands and playable cards in the format - rarely is there a reason for a deck that splashes for one color to necessitate a manabase that outnumbers the dual lands in the format. An example circa 2018-2019 is Modern Bant Spirits; despite being a majority White-Blue deck (White-Blue Spirits being a rival deck in the same format), the four Noble Hierarchs (plus Collected Company) require a high number (fifteen) of untapped green sources to support it out of a 20-land manabase. In contrast, the White-Blue version has a similar number of white and blue sources but with the space for three utility lands in a 21 land manabase, as well as one fewer painland, fetchland and shockland, lowering the life lost over a game.
Manipulating the color balance
Besides finding a perfect mix of colors and mana generators there are plenty of ways to twist the balance even further.
- Artifacts: Cards needing no color cause no color problems.
- Hybrid cards: The fact that these cards may be played from any mix of its hybrid colors mean that it is very easy to play such cards, especially if the deck is only two-colored.
- Morph: The ability Morph enables some cards to be played without the use of colored mana. Later they can be activated by the relevant colored mana.
- Multicolored: Few people realize this, but in a deck with two colors and an even amount of the two colors of mana it is far more likely that you can play a multicolored card (Example: Meddling Mage) before you can cast a full-colored card (Example: White Knight). The reason for this is purely statistical. Using such knowledge may press the color balance to the edge of what is mathematically possible.
- Split cards: Increases the chances of casting a spell for the available mana. If the first card of the "split" cannot be cast, there is always a chance that the second may.