Hello Support,
I cannot find where I can change the font color of submenu in products.

There are only few options for font color change in Customer.

Thank you for your support.

Hi there,
please add this css code to your css/custom.css

.uk-nav-navbar > li > a {
color: #ff0000;
}

Sometimes its easylier to add a css code than search into the customizer.
How to find the right css style:
www.joomlaplates.com/template-customizing