To apply a color to a single menu item, first get the ID or the menu item.

You can find this in your back office, and go to where you manage your menu's. The last column holds the id's

Get the ID, and then go to the custom CSS tab in the joonextpro template and add the following code

#bootstrap_a_menu_item_184, #bootstrap_a_menu_item_184 > a {
color: red !important;
}

where the 184 must be changed for the ID of the menu you want to have changed.