Volume pricing

Each variant of a product is considered as its own product. so you can manage the stock for each variant and the minimum quantity of prices apply to each variant and not globally.
That's why you have such result.
If you don't need to manage specific variant information (like a different image for each variant for example), you should use a custom item field (you can do that by creating a custom field of the table "item" via the menu Display->Custom fields) instead of a characteristic. That way you will only have one product but you will still be able to ask the user the color he wants.