Everything regarding the appearance (skins, attachments and slots, color, etc) of the skeleton can be set via SkeletonAnimation.Skeleton
:
SkeletonAnimation skeletonAnimation = this.GetComponent<SkeletonAnimation>();
skeletonAnimation.Skeleton.R = red;
skeletonAnimation.Skeleton.G = green;
skeletonAnimation.Skeleton.B = blue;
skeletonAnimation.Skeleton.A = alpha;