Use an Item Renamer

Importing…/warp how2rename

Go to https://www.birdflop.com/resources/rgb/ or https://minecraft.menu/minecraft-rgb-generator 

Use the menu there to create your custom item

Copy the code that it gives you (the stuff in the red rectangle in the above screen shot)

Go back to Mineseed

Make sure that you have the item renamer in your inventory and make sure you are holding the item you want to rename

Do /rename code (paste the code there)
/rename &#FFFFFFE&#FFFBF0x&#FEF6E1a&#FEF2D3m&#FEEEC4p&#FEEAB5l&#FDE5A6e &#FDDD88T&#FDD97Ae&#FCD46Bx&#FCD05Ct &#F7B268G&#F5A36Do&#F29473e&#F08579s &#EB6785H&#E9588Ae&#E64990r&#E43A96e

Enjoy!