@Pharan Thank you very much. I did not know that I had to write "using Spine.Unity;" to use these extensions. I guess I am in mixed with C++ mindset and I use things like "using SkeletonAnimation = Spine.Unity.SkeletonAnimation;" to avoid importing the whole namespace.
C# lesson of the day learned!
Btw: having skeleton.r, g, b, a is not the best API in my opinion. There could be something like skeleton.color or at least skeleton.red, blue, green, alpha.