There is actually a span element preceding the checkbox which handles the visual display of the checkbox. The easier way to change the color is through Theming, though this would change all checkboxes, buttons, etc.
The HTML element you actually need to modify is
span:has(+input[type='checkbox'])
However, if you want to override it as a one-off, you’ll have to inspect the element and identify the internal classes applied for when it is selected or not so you can apply color correctly. Since it’s not officially supported, those classes may change from version to version, and even the HTML object I’ve identified above may change.