Shaders are items used by Guardians to customize item colors.
In Destiny, Shaders can be used alter the color and texture of the player's currently-equipped armor set after reaching level 20. They have unlimited uses, but only one Shader can be equipped at a time. Shaders are obtained from vendors in the Tower, the Reef, and the Iron Temple, or as activity rewards.
In Destiny 2, Shaders alter individual pieces of armor and Jumpships, but are single-use consumables. They can be obtained as activity rewards, but are shared between all characters. With the introduction of Destiny 2 patch 2.0, players can dismantle Shaders in stack of 5 in Master Rahool's vendor screen, as opposed to the previous one-at-a-time dismantle from the player's own inventory.