Using the loop for portfolio

Hi! Im using wordpress as a CMS, everything is perfect, I styled the main menu li class “current_page_parent” to be like “pressed” when you’re inside it

I have two loops, one in the “blog” page and another in the “Portfolio” page, the only problem is that when I hit “Read more” in a post from the Portfolio page and I enter the “current_page_parent” goes to “Blog”

I have created two different post templates, I have no idea how to force the “current_page_parent” class to stay in “Portfolio” when I enter a post in that category

I think this can be accomplish using php but i have no idea how to make that if you could help me I’ll be very thankful