Lately, I've been playing a lot of Spinter Twins.
After Treasure Cruise and Dig Through Time got the axe in Modern, Splinter Twin has become an even larger staple of the format. If you're unfamiliar with Splinter Twin, the deck's goal is to combo off with a creature like Pestermite and then cast Splinter Twin on it. You tap the original to make a copy, which untaps the original, which makes another copy, repeat, and attack them for lethal.
What's make the deck particularly potent is that not only is it a simple two card combo, it also gets to threaten the combo all the time. If you tap out on your turn your Twin opponent can flash in their Deceiver Exarch and combo on their turn. This forces your opponent to play awkwardly, usually keeping mana open for removal spells, and leaves them vulnerable to a more tempo-oriented gameplan. With Twin becoming an ever increasing staple of the latest Modern season, many of the lists are branching out into the third color in order to either one-up the mirror or pad the numbers in other match-ups.
So let's examine some of the cards that these splashes get you access to.
RUG - Temur Twin - Tarmo Twin
Splashing Green is perhaps a little deceptive because most of the UR lists are floating a single Stomping Ground for Ancient Grudge out of the sideboard, but the card that decks more heavily splash Green for is Tarmogoyf. The "Tarmo Twin" deck actual got quite a bit of snide comments thrown at it about smashing Tarmogoyf in every deck, but everyone's favorite $200 bruiser fits very nicely into the tempo plan. Tarmogoyf forces interaction, it's too big to be simply ignored, and this leaves you open for the combo. Other cards that sometimes get a nod are Huntmaster of the Fells - which I think is still one of the coolest named cards in Magic - and Thrun, the Last Troll. Blue and Red have you fairly well covered for spells, so Green typically is just bringing bigger resilient threats.
URB - Grixis Twin - Tasigur Twin
Similar to the role Tarmogoyf plays for the Green splash, Tasigur, the Golden Fang gives you a cheap threat that must be dealt with, though it does require a little more investment to build around it - often some number of Thought Scour. It doesn't take that crazy of the hand to power out Tasigur, the Golden Fang very early between fetches and all the cantrips or simply play it out later with plenty of mana open for the ability or to fight over it. Tasigur, the Golden Fang also gets to dodge Abrupt Decay, a card otherwise very strong against the combo, and his ability lets you grind out games or be a little more liberal with the combo. Black also opens up access to discard, Inquisition of Kozilek and Thoughtseize along with hard removal like Terminate. And let's face it someone, somewhere, has probably tried to make Cruel Ultimatum Twin work.
URW - Jeskai Twin - Partiot Twin
And finally, my personal favorite is splashing some Plains into those Islands and Mountains. Of all the splashes, UWR has perhaps been most deeply explored, mostly since it Venn diagrams into another Modern deck - UWR Control. You can spend a lot of time tweaking the ratios of how much you tip one way or the other. Sometimes you're UWR Control that's using the combo as a win condition, or hell slap Splinter Twin onto Wall of Omen to build your own personal Howling Mine. Or maybe White is just a smaller splash for sideboard card, White has a bunch of awesome ones, and Restoration Angel. Restoration Angel gives you another three power flash flyer to keep on the tempo plan. It also dodges Lightning Bolt and Abrupt Decay, and it can still combo with Kiki-Jiki, Mirror Breaker, which can kill through an opposing Spellskite.
I'm a big fan of UWR right now because you can side out the Splinter Twins completely and dodge a bunch of the common cards used against you, and Choke has increasingly become Green decks hate against you and so having access to Tear comes in handy.