This seems to be with the latest update in Unity 2017.3 adding the attribute "[StructLayout(LayoutKind.Explicit)]" for "Color32". This forces the declaration for Color32 in the code.
While we wait for the unity spine plugin to update, we can initialize the variables using Color32 in the scripts. In my case I'm using Spine Animator and Animation, and change in the "SpineMesh.cs" lines 460 and 664 with:
Color32 color = new Color32();
I did that and got it to work. Hopefully spine can address this issue with this simple fix.
Hope it works for you