Skins

Skins are unlockable models available for units and structures in StarCraft II and StarCraft: Remastered.

StarCraft II
Initially only available with collector's editions, skins have gradually been added through other means. With Patch 3.7 of Legacy of the Void, skins are now available for purchase through the StarCraft II client. Skins will not be too "crazy;" regardless of the skin, it is intended that players are easily able to identify which unit is which. In comparison to Heroes of the Storm, StarCraft II skins will be more grounded, and intended to be in sync with the setting.

As of BlizzCon 2016, there are no plans to disable skins for competitive multiplayer.

As of Patch 3.12, only four skins at a time per race may be used in games with four or more players. These can be set in the "large format games" section of the skins tab.

War Chests provide a full set of racial skins for each race. Season 1 of War Chests provided a Special Forces skin set for terran, a Leviathan Brood skin set for zerg, and a Forged skin set for protoss. Season 2 provided Purifier, primal zerg and Mira's Marauders skin sets. Season 3 included Umojan skins for Terran, Simulant skins for Zerg, and Golden Age skins for Protoss.

Several other skin sets have been storyboarded, including Mecha sets for terran, Shakuras, Judicator and Frost for protoss and Aquatic, Feral and Jungle for zerg. Building skins will be introduced in the fourth war chest.

Skins are not usable in Co-op Missions, as many skins are made to represent separate units in the mode.

BlizzCon 2017
Join the battle in style with these skins from the BlizzCon 2017 War Chest. Recruit the Terran Dominion soldiers, assimilate with the Zerg Leviathan Brood, or add the Protoss Forged warriors to your army today!
 * Suit Up and Roll Out

With unparalleled technology, the elite Terran Dominion Special Forces are often brought in to make the best of a bad situation. With the might of the Special Forces inspiring your every move, the enemy won’t know what to expect.
 * Call in the Dominion Special Forces

The unpredictable Zerg Leviathan Brood overtakes anyone who gets in their way—and your enemies will definitely be taken by surprise. With this spine-covered strain in your Swarm, you’re sure to creep up on your opponent.
 * Spawn the Leviathan Brood

The most cutthroat faction of the Protoss, the Tal’darim—also known as the Forged—take no prisoners. These zealous warriors are ready for vengeance. Add them to your forces.
 * Take Control of the Tal’darim

Katowice 2018
Outfit your army with these skins from the Katowice 2018 War Chest. Intimidate your enemies as the mercenaries of Mira’s Marauders, prey upon the weak as the Primal Zerg, and command the dangerous machine forces of the Protoss Purifiers.
 * Enter the Battlefield in Style

Mira’s Marauders are a reckless and rowdy crew led by the fierce terran mercenary Mira Han. Intimidate your enemies with heavily modified weaponry, sun-weathered armor plating, and lots and lots of spikes. If you want something shiny, look elsewhere; these mercenaries play dirty and don’t waste time polishing their gear.
 * Call In Mira’s Marauders

Before they assembled into the Swarm, the first zerg were primal—monstrous creatures born from the Planet Zerus, spawned for one single purpose: to collect essence and evolve. Inspired by Dehaka, the Primal Zerg pack pays homage to the Swarm’s ancestors by fitting your units with the heavy chitinous plating and primordial tusks of their forbearers. Equip the skin pack and begin feasting on the essence of your enemies. Survive. Adapt. Evolve.
 * Absorb the Essence of the Primal Zerg

Built by order of the Conclave, the robotic Purifiers are an elite fighting force modeled after some of the protoss’ fiercest warriors. Equip these Purifier skins and take control of an army that was deemed too dangerous to remain active—until the time came to drive Amon back into the Void. This sleek and sophisticated set gleams white, a nod to Fenix’s army.
 * Ally With the Purifiers

BlizzCon 2018
Mission Briefing The Shadow Wars saga continues with three brand-new comics, as conflict between disparate forces of the terran, zerg, and protoss reaches a boiling point on the fringes of the Koprulu sector.

Every soldier in the Dominion would love to get their hands on some shiny new tech from Umoja. And who can blame them? With their slick white designs and highly-advanced features, the Umojans make quite the impression—even before they fire at you.
 * Join the Umojan Protectorate

The Simulant Project was a mysterious initiative undertaken by a group of commissioned scientists. They created reproductions of various zerg specimens using entirely terran technology (or so they said). Whether they were successful or not is anyone's guess, as the whole team unexpectedly vanished…
 * Discover the Secret Simulant Project

Templar from the Golden Age wore brilliant armor to convey their status and honor as warriors. After years of suffering the dark consequences of the End War, the protoss' triumphant past burns even brighter in their memory, and the Templar have proudly revived their ancient raiment.
 * Recall the Glory of the Golden Age

StarCraft: Remastered
A trio of skins are available for those who pre-order StarCraft: Remastered. They consist of the "Korhal command center," the "Char hive" (the hatchery and lair also have the skin) and the "Aiur nexus." The skins are available for use in the campaign. They can be toggled on and off through the game's option menu.

Skins for units have not been ruled out, but an issue exists in their implementation, as it would require the developers to make a new model for each skin in 3D, then convert it into 2D. In terms of time investment, this would mean effectively creating a single model twice.

Development
When designing a skin set, the developers brainstorm a bunch of ideas, and then narrow it down. What's considered is whether the skins fit within the StarCraft universe, and whether they fit the race itself. One manner of applying skins is to take the original unit, do a paintover, and then apply it. The upcoming aquatic zerg skin set is an example of this, stemming from the idea of what would the zerg be like if they had evolved underwater. Often the colors of the new skins have to be toned back a bit, given the considerations of team color within matches.