As seen in the picture below (with only white and black as predefined options), there are a couple of issues with the interface. For one, the custom hex field relies on the placeholder parameter value to indicate what the field is for, rather than a proper <label>. More importantly than that though, the opacity field starts out with "100" and no indication of why that number is there or what it's for... not even in a placeholder.
As seen in the picture below (with only white and black as predefined options), there are a couple of issues with the interface. For one, the custom hex field relies on the placeholder parameter value to indicate what the field is for, rather than a proper
<label>
. More importantly than that though, the opacity field starts out with "100" and no indication of why that number is there or what it's for... not even in a placeholder.