Hi,
Virtuemart core and the custom field plugins are responsible to generate the custom fields.
Our template only styles the elements to increase the user-exerience. This is actually what the templates do.
What you are saying is "it is what it is" and it is a Virtuemart+CustomField plugin thing. If you use (VM Custom, Stockable Variants) option, there is no "Select Color" option in the drop down. So, the 1st option will be selected automatically. But if you use the (Custom Field: Generic Child Variant), then the automatically selected option would be the "parent" item, which is "not order-able" so the add to cart button won't work until you make your selection from the drop down.
There is nothing can be done about it beside Virtuemart Core Hack to CustomField plugin PHP file. Virtuemart Hack is not supported (because eventually your changes will be gone if you update your VM to any newer version) and also it is beyond our free support.
Check
forum.virtuemart.net if you are interested in Virtuemart coding Hack.
Regards,
Support