Hello
This may vary depending on your template but in most cases to amend you can add the following to the Custom CSS field of your templates parameters (Extensions -> Templates -> [YourTemplateStyle] -> Custom CSS)....
(edit to taste)
.hornav ul.menu > li.active > a {
color: red;
}
To edit the hover color you can also add the following (edit to taste)..
.hornav ul.menu > li > a:hover,
.hornav ul.menu > li > span:hover {
color: #000;
}
Ciaran