Undercoats are more important than most people realise. Undercoats are designed to even out the surface specifically to give you an even surface to paint your coloured matt or eggshell emulsion over. So we use undercoats for more consistent colour.

Primers are for bare plasters and tricky surfaces to act as bridge to help the topcoat (or undercoat) adhere. 

Want to know more: What is the difference between an undercoat and a primer?

Already painted walls with patchy colours or inconsistent colours? Use an undercoat. Caveat. In reality, for modern paints like Auro 555 emulsion you can paint these straight over most existing paint on your walls with the normal 2 coats and it should be fine, maybe use undercoat when you are painting white over a dark existing painted surface.  

Bare plaster? Tricky surfaces? Use a primer. Fortunately the undercoats and primers we have act pretty much the same way, so panic not!