You should be able to use a body class to style your menu item :
.single-project .menu-item-45 a
In “Appearance > Menu”, you could also add custom CSS classes to menu items. You could give your Portfolio menu item a class of “portfolio” and do this :
.single-project .menu .portfolio a
I do this mainly because I don’t like .menu-item-[number], it’s more readable with a plain text name, but the first solution should work anyway.