u/No-Pomegranate53

I don’t know how to pick my underpainting color and don’t understand the principle. I tried using completely complimentary colors- like purple under a yellow ish painting, and it came out looking odd. I need a book or course rec for this. I also googled what the Old Masters used and obviously, each painter had their preferred underpainting… but I just want it to make sense.
I also am not a big fan of the neon pink that some instagram artists use because it gives the painting that crazy bright look.
Just looking for good theory.