Creating hair can be tricky because of possible transparency issues.
Haircap is an additional texture applied to the back of the head to reduce visual artifacts caused buy transparent hair textures of limited resolution. For any asset you can choose whether it should force a particular haircap to turn on or not. This is usually important in hairstyles with head skin visible such as bald hairstyle, mohawk.
If you want to use your own haircap -- let us know.
Avaturn lets users to recolor hair. When uploading an asset you can choose whether this functionality should be enabled or not. Recoloring works by mapping grayscale diffuse texture into the user-selected color. In general when coloring algorithm applied -- darker gray values in diffuse map translate into darker colors, and light gray in diffuse map translates into lighter color.
You can create hats without hair. In this case you can follow clothing pipeline and define an alpha map.
Mixing hair with hats
It's totally fine to mix hair with accessories such as hats, ribbons and other objects. However you might want to disable recoloring for accessories. You can do it by including no_ramp into the material name of the accessory mesh.
Note: if you want to disable recoloring on all meshes, there is no need to include no_ramp in the name of each material. You can disable it in UI when uploading the asset.