Hi Peter,
The template.css file also uses an extra class name, which, for the browser, is "stronger" than the override from Custom Style Preset settings. In that case, don't waste time, and just use a simple Custom CSS to set RED color
.fixed-menu #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item.active > a,
.fixed-menu #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item:hover > a,
.fixed-menu #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item:focus > a,
.case-studies-page.view-dynamic #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item.active > a,
.case-studies-page.view-dynamic #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item:hover > a,
.case-studies-page.view-dynamic #sp-header #sp-menu .sp-megamenu-wrapper .sp-megamenu-parent .sp-menu-item:focus > a
{color: red; }