Chroma skins are recolored variants of a champion's classic or purchasable skins. Each Chroma is available for purchase individually for 290, as well as part of a bundle known as a Chroma Pack at a discounted price. Chromas go on sale for 2000 or 195 each during the Essence Emporium.
The first Chroma skins came live with patch V5.8 on April 28, 2015 in Season Five.
Some chromas are special than others:
Golden: all Golden Chromas are a part of events and only unlockable through the Hextech system. Most of them will be listed in the Loot Exclusive section.
Ruby: since V8.15, these chromas are only unlockable by buying the chroma bundle.
This has not been the case as the first instance of these chromas being bundle exclusive goes as far as V8.3.
Emerald: since V8.20, these chromas are part of the Partner Program (with some exceptions). For more information, see the Partner Program Chroma section.
These chromas are part of the Partner Program, a promotional scheme wherein content creators from around the world were given codes that they can distribute to their viewers that unlock the chroma as well as all necessary content to use the chroma (i.e. the champion and skin). To redeem the code, you have to follow these steps.
Partner Program Chromas are also available from the Essence Emporium for 10000 each, but only once and only after at least 6 months has passed since their release.
The first Partner Program Chroma distributed was for Divine Sword Irelia, but this Chroma was not given the exclusivity that would become the standard - and is available in-store as with any other Chroma. Since then, all subsequent Partner Program Chromas have been exclusively available from the Partner Program and one-time Essence Emporium sale.