Many thanks for your suggestions and help.
I'm not advanced in programming but I can edit html and css without problems.
I found a simple way to do that:
Catch the category class of the on-screen project and change the colour of the desired menu item. Like that:
.category-4 .menu-item-551 a {
color: #ee1c25 !important;
}
From coding standpoint might not be perfect but worked for my needs.
Best!
Marc