enhance child products dropdown menu

[positronio]

hello i have joomla 3.6.5 and virtuemart 3.2.1. The site url is www.havaianas.com.gr. I would like in the dropdown menu of the child products, the numbers that are out of stock to have different color. For instance gray. Or i would like the shoe numbers to be shown like in the attached photo. In the attached photo the out of stock numbers are shown with gray color

thank you in advance.