To change color of Navigation on Portfolio Overview Page only, you can follow these.
#1. First, find Portfolio Overview Page ID.
In my example, it is: #collection-673e9a14857d4032d3e2f27b
#2. Next, use this code to Custom CSS box.
#collection-673e9a14857d4032d3e2f27b {
div.header-nav-item>a {
color: #f1f !important;
}
div.burger-inner>div {
background-color: #f1f !important;
}
}