I have got the same problem, the default exported spine sprite has lower saturate, and 4x bigger size, you can see as below

and if toggle the "Alpha Is Transparency" option on, some glitch appeared.

I found this issue only appear in linear color space.
An old forum thread mentioned this, the user gumboots provide a workaround.
[http://esotericsoftware.com/forum/Colour-different-Spine-Unity-colouring-in-Unity-10586?p=48258&hilit=sRGB#p48258]
If the patch could be merged into the official Unity runtime, may release some future pain :-)