Add color to part of a mesh or OBJ Qt3D 5.9 in QML

In Qt3D 5.9, I am using scene 3D to render an .obj file and display it. I also have enabled object picking, so when a user selects part of the object, I know exactly where on the model they clicked. What I would like to do is add color to that part of the obj/mesh that the user clicked on. To be more specific, for the 'y' value that the user clicked on, I want to color a line all the way around the object model on that 'y' value. I've looked around online and can't find anything to help. Unfortunately I'm not familiar when it comes to 3D objects, meshes, etc. How can I color just part of a mesh in Qt 3D 5.9?