That requires specific shaders.
The current setup does tint according to a standard Multiply algorithm. (as in similar to the Multiply transfer mode in Photoshop). This is pretty standard for games because it's arguably among the fastest algorithms since all it has to do is multiply numbers.
I'm not terribly familiar with how to program shaders or how it'd be put in libGDX.
But if you decide to go that route, you would need to deal with extra things like batching and layering. This has less to do with the Spine system and more with rendering code.
