As described, exporting a project with any slot set as Tint black and reimporting it (JSON or binary)

On JSON, the log will return

Cause: [npe] Cannot assign field "r" because "<parameter2>" is null
   at com.badlogic.gdx.graphics.Color._(_:361)

To reproduce:
Open any clean project
Set a slot as tint black
Export
Import

I tried it on Spineboy just to be sure.
Not a problem for me, just caught my attention so I isolated it for a report