Shaders
Blue! No, yellow!
Shaders are items you can find in the world or aquire by trading with villagers.
They don't have a purpose by themselves, but they can be equipped to <link;revolver;Revolvers>, <link;mining_drill;Drills>, <link;chemical_thrower;Chemical Throwers>, <link;heavy_plated_shield;Heavy Plated Shields>, <link;railgun;Railguns>, and <link;accumulator_backpack;Accumulator Backpacks> (and possibly other things you still have to discover) to change the designs of the item.
Simply put the item in an <link;engineers_workbench;Engineer's Workbench> and equip the shader in the appropriate slot.
You can also apply them to a Minecart, Banner or a <link;balloon;Balloon> by rightclicking!
For a full list of all available shaders, their looks, references and more details, <link;shader_list;check here><np>
<&grab_bag>Shader Grabbags are items containing a random shader.  They can be found in crates, traded from villagers or dropped by bosses.
The rarity of the contained shader is equal to, or better than the rarity of the bag.
When using a bag, you are more likely to get a shader you do not yet own than one you already possess.
Shaders bags can be crafted into two bags of a lower level. This will lower your chances of getting rarer shaders but provide you with more attempts.
