Скачать или смотреть How to Get Coloured Custom Names in Minecraft Java 1.21.5

1 colour:
/give @p minecraft:itemhere[minecraft:custom_name=[{text:"here",italic:false,color:"#FF0000"}]]
2 colour:
/give @p minecraft:itemhere[minecraft:custom_name=[{text:"here ",italic:false,color:"#FF0000"},{text:"here",italic:false,color:"#FFFF00"}]]

Example:
/give @p minecraft:diamond_sword[minecraft:custom_name=[{text:"OP",italic:false,color:"#55FF55"},{text:"Sword",italic:false,color:"#5555FF"}]]

Black: #000000

Dark Blue: #0000AA

Dark Green: #00AA00

Dark Aqua (Teal): #00AAAA

Dark Red: #AA0000

Dark Purple: #AA00AA

Gold: #FFAA00

Gray: #AAAAAA

Dark Gray: #555555

Blue: #5555FF

Green: #55FF55

Aqua: #55FFFF

Red: #FF5555

Light Purple: #FF55FF

Yellow: #FFFF55

White: #FFFFFF