Hi,
You could create a custom field of the table "item" for that. Make it of the type "radio" so that you can have have HTML in each value title to display an image with a color or something. And with the display options of the custom field, you can configure it to not display on the frontend and emails, and only on the backend views.
That way, when you edit a product of an order, you'll be able to select the value you want in the custom field of the edit popup.
Now, this custom item field value won't appear on the orders listing, in the "products" column. This would require some customization of the code in the order / listing view for your backend template via the Display>Views menu of HikaShop.