Mardu Midrange (Standard)

A Burn deck at its core

-Red runs the clock and controls the board if necessary through burn:
Incendiary flow's exile ability comes in very handy against delirium/emerge/Oujutai's Command strategies.
Collected Defiance often provides 2 for 1's against creature based strategies especially Bant. Additionally, its hand cycle ability had nice synergy with duress/transgress vs. Emrakul/ Emerge decks

Red also provides CA and the finishing power through Chandra Planeswalker and GDD + Bedlam Reveler which are also usually game changing targets for Nahiri's ultimate.

-White: Speaking of Nahiri, she is the reason to go 3 colors. Adds a level of versatility the deck previously lacked. She owns enchant strategies, including izzet machine gun, and can take care of any troublesome creature not named Advocate/Avacyn. The filtering allows you to search for the right spell, sets up the grave for GDD/ Reveler and adds a crazy amount of loyalty for the opp to deal with. Discarding fiery temper is icing on the cake.

-Black is used mainly to have more game against emrakul decks (transgress and duress out of sideboard). It also helps against aggro/bant (languish) and additionally provides CA (read the bones) and Shambling Vents in grindy MU's.
probably should go to 4 considering how often the board is empty.