I brought that earlier, as soon as you buy it, it takes you to an area that says configure. Once you click that, it asks for the hex code or colour.
I may of got it wrong, but I think that's what you do.
I've returned your bells so that you can buy the item again. You can just discard the one that is currently in your inventory.
When you configure it, make sure to use a hash tag before the hex code e.g.: #000000. If you have done it correctly you will see an automatic preview of it on your page. Do not confirm your choice until you see that it is working!