Can you use any paint as an undercoat?

You cannot use any paint as an undercoat, as undercoat and primer have specific formulations designed to seal, bond, and prepare surface for topcoat, while regular topcoat paint is designed for final color and protection. Undercoat is typically high solids, good at filling, sealing porous surfaces, blocking stains, and providing adhesion. Using random topcoat paint as undercoat may lead to poor adhesion, uneven absorption, tannin bleed, and peeling. However, there are some exceptions, you can use self-priming paint which is marketed as paint and primer in one as both undercoat and topcoat if surface is previously painted and in good condition and similar color, as it has primer properties. You can also use flat white latex paint as makeshift undercoat in emergency for walls, but it will not seal as well as proper primer and may require more coats. For bare wood, bare drywall, metal, or stained surfaces, you need proper primer like wood primer, drywall primer, metal primer, or stain blocking primer, not just any paint. Using wrong type like oil paint over latex without bonding primer will fail. So while any paint may provide some base, it will not perform like true undercoat, and for best durability you should use product labeled as primer or undercoat suited to surface.