You can change the color in the Responsive section of the CSS around line 3830 –
.site-header .genesis-nav-menu.responsive-menu .current-menu-item > a,
.site-header .genesis-nav-menu.responsive-menu .sub-menu li a,
.site-header .genesis-nav-menu.responsive-menu .sub-menu li a:hover,
.site-header .genesis-nav-menu.responsive-menu .sub-menu,
.site-header .genesis-nav-menu.responsive-menu > .menu-item-has-children:before,
.site-header .genesis-nav-menu.responsive-menu > li:hover > a,
.site-header .genesis-nav-menu.responsive-menu a:hover {
color: #fff;
}
You can start by changing it to color: #333;
or whatever color you want it to be. That should change it when the site is being viewed from a mobile device or any browser under 800px width.
Let me know if that works.
The site looks great! Let me know if/when it’s done so I can share it.