Some of the best advice I've heard with regards to deck building in Khans of Tarkir Limited is to consider your deck two colors, with a splash of a third or fourth color when reasonable (possibly something said on Limited Resources? Citation needed). So let's discuss briefly how splashing works, what kind of mana we need to do it, and what kinds of cards we might want to splash or avoid splashing.
As far as mana bases go, in general, the absolute minimum number of sources one should be playing for one's main color is eight. Nine is even better, especially if you have a lot of cards that require two colored mana, and ten or more will really make your deck comfortably consistent. If your list is relatively even between two colors, that means you'll need at least eight sources of each of those two main colors, which doesn't leave a lot of room for splashes in a deck with just basics.
As for splashes, I personally like to have three sources if I'm splashing a single off-color card, and more like four or five sources if I'm splashing more than one. This means that at bare minimum, for a two-color deck with a light splash, you need eight sources of two colors and three sources of a third, adding up to 19 total sources. Limited decks with 19 lands are more likely to flood out, and in a format featuring three-color cards, your spells are likely to be even more demanding than two colors with a light splash. That makes fixing in Khans of Tarkir absolutely essential.
If you want to be greedy with the spells you're playing, you need to prioritize the refuges and trilands when they come to you. If you do take a bunch of fixing, the question becomes: what kind of cards are good to splash? Let's look at a few examples.
Assuming you are only splashing one of the colors, you could do worse than splashing the charms. Some are better splashes than others, with something like Mardu Charm likely being better than the more reactive Temur Charm. Because these are generally good both early and late, it's not a disaster if you don't draw your splash color right away.
Powerful off-color morphs are good splashes because even if you don't draw the splash color right away, you can still play them face down. If your deck is really hurting for playables, you can even play morphs that you literally cannot flip, although that's not exactly a plan to go in with. Morphs are an especially good splash if your mana base is strained a little bit, since they're not completely dead if you don't draw a needed color.
Assuming you're splashing powerful spells and not sub-par junk, removal is a great splash. Although it may be annoying to not be able to play a removal spell early, if your deck is built in such a way to help you survive to the late game, being able to pick off your opponent's bomb can be game-winning.
Splashing your win condition or cards on the top end can be fine if you're okay with not necessarily casting them on curve. Even though it costs three mana, something like Savage Knuckleblade probably qualifies because it doesn't hurt to have mana up for its activated abilities when casting it. Game enders like Flying Crane Technique are easy splashes.
Splashes to Avoid
Major things you should avoid splashing are:
- Low-cost, low-impact creatures that are only good in the early part of the game.
- Cards that require two mana of the splashed color (unless your mana base is insane).
- Cards that require two splashed colors (unless, again, your mana base can handle it. You need to be sure!)
- Reactive spells, especially counterspells or combat tricks—if you don't have the mana when you need them, they're useless.
This was a very general discussion about a very complex topic, but if you're having trouble figuring out what cards are worth splashing in this format, I hope this was helpful. May you open bombs and win lots of packs!