Hi
It's inheriting the settings for link color of main menu in template >> Theme color settings:
If you want to customize it, you can add this custom CSS code:
.t4-megamenu .dropdown-menu li > a:hover, .t4-megamenu .dropdown-menu li > a:focus, .t4-megamenu .dropdown-menu li > a:active, .t4-megamenu .dropdown-menu li > .nav-link:hover, .t4-megamenu .dropdown-menu li > .nav-link:focus, .t4-megamenu .dropdown-menu li > .nav-link:active, .t4-megamenu .dropdown-menu li .dropdown-item:hover, .t4-megamenu .dropdown-menu li .dropdown-item:focus, .t4-megamenu .dropdown-menu li .dropdown-item:active {
color: your_color_here !important;
}