I just tried the following code successfully:
var slot = skeletonAnimation.Skeleton.FindSlot(slotName);
slot.SetColor(new Color(1, 0, 0, 1));
If you don't see any effect, please check the script update order as described above. Your script shall run after SkeletonAnimation and SkeletonRenderer. If this is already the case, please post the code you are using to set the color.