I just noticed something in one of my 700ml bottles of selenium shade 4 (seeing it in shade 2 bottle too) that the top empty area is turning bright blue. I know that the inks need to be shaken from time to time and I always shake the bottle gently before filling carts, but have never seen this before. I have a photo of it but it will not take it as an attachment to this post.
Is this something I need to worry about, like inconsistent color(s)?
Not to worry as long as you keep the ink shaken. You are witnessing thin layer chromatography, one of the color pigment base has a different bleeding speed (migration) on the plastic HDPE bottle. This is why you saw the blue color around the plastic bottle. A chemical we used to use is no longer available and the new chemical which actually gives us better encapsulation advantage and print head performance causes a more perfect separation of the components. So just keep the ink shaken from time to time.