Use an Item Renamer

/warp how2rename

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

2. Use the menu there to create your custom item

   

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

4. Go back to Mineseed

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

6. 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

7. Enjoy!