Rank 10?
More or less a theory. I’m on chapter 1501.
I just noticed that the color from the start of being a gu master (rank 1) corresponds to the start of being a Gu immortal (rank 6) and follows up to rank 9. While rank 10 is not represented in the highest rank of mortals (5).
Rank 1: Green Copper - Rank 6: Green Grape
Rank 2: Red Steel -Rank 7: Red Date
Rank 3: White Silver -Rank 8: White Litchi
Rank 4: Yellow Golden -Rank 9: Yellow Apricot
Rank 5: Purple Crystal -Rank 10: Purple ???
No idea if this is simply how GZR built the world and intended it to be but there could be a rank 10 with for example purple plum immortal essence. This would follow the trend of 1 and 6; 2 and 7; 3 and 8; 4 and 9 each corresponding to the same color.
The mortal primeval essence is also described with minerals or metals vs the immortal essence being described as fruits. So maybe I’m right (probably not).
I’m not the sharpest tool in the shed so don’t take this too seriously.