Painter
The painter is how you change the texture of a model. It offers a few brushes, each with a specific role.
“Fill Surface” Brush
This brush colors one surface of a model. A model is made of 1 or more surfaces. Each surface is made of 1 or more faces (triangles).
activate the brush
select a color
click the surface you want to color

“Fill Model” Brush
This brushes does the same thing as the surface-fill brush, but for all surfaces of the model at once.
Sticker Tool
This tool lets you add 2d assets to a UV-unwrapped[1] model like stickers:

[1] a 3D resource with no UV data will not be able to receive stickers. UV unwrapping is on our backlog, but until then, use blender or another tool to save the resource with UV data.
Notice the tool shows details of the stack of stickers currenlty applied on a model when it’s clicked:

Stickers can be shown/hidden, renamed, moved up/down the stack (higher means it’s displayed “on top” of others), removed, and locked. A locked sticker cannot be manipulated.
Manipulations
To manipulate a sticker, use the mouse:
left press + release (without moving): select the sticker
left click + drag: move the sticker on its surface (a sticker cannot move from its original surface to another)
right click + drag: rotate
mouse wheel: scale

Export
When exporting the asset, GAC blits all stickers onto a single texture applied to the mesh.