With the new Anvil Color Codes, Exclusive rank and above can now use normal color codes, as well as HEX color codes to rename their items.
Below are some examples, which will show you how to rename your items successfully using HEX codes.
# NAME: "HEX"
FF: "fad390" #Flat Flesh
SQ: "f6b93b" #Squash Blossom
IP: "fa983a" #Iceland Poppy
CO: "e58e26" #Carrot Orange
MM: "f8c291" #Melon Melody
MR: "e55039" #Mandarin Red
TR: "eb2f06" #Tomato Red
JR: "b71540" #Jalapeno Red
LI: "6a89cc" #Livid
AB: "4a69bd" #Azraq Blue
YG: "1e3799" #Yue Guang Lan Blue
DS: "0c2461" #Dark Sapphire
SP: "82ccdd" #Spray
DU: "60a3bc" #Dupain
GS: "3c6382" #Good Samaritan
FB: "0a3d62" #Forest Blues
PG: "b8e994" #Paradise Green
AG: "78e08f" #Aurora Green
WA: "38ada9" #Waterfall
RE: "079992" #Reef Encounter
If you require further support using HEX codes on anvils, you can open a support ticket here: https://discord.com/channels/250113771647926273/513675986668355614/1038410198102069288