To open the item editor run the command /itemeditor, if you want to edit an existing item run /itemeditor <itemName>
Now you can customize your item. You can use colors codes for the customization and for minecraft versions that supports hex colors you can use hex colors too with the same prefix '&' like that "&#RRGGBB" basically "&#<hex code>" and for noraml codes you need to do "&<code>" like always
