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Primer is not usually a term applied to wall paint (emulsion). There are a few high solid content emulsions that are useful for colour changes. Dulux Trade Supermatt used to be my choice. I use a specialised ceiling paint from a local supplier now but I can't remember what it's called.
If on the off chance you are talking about paint for woodwork then Zinsser 123+ is a good primer. Don't forget to stir the paint in the tin before you use it and keep stirring it as you go along. The solids settle very easily.
Sometimes there's a good reason to use it on walls but it works out pretty expensive.
Helpful people of the DIY thread, I'm after recommendations for a primer to paint over a darker colour. Any recommendations? Top coats will be a white Dulux Professional Matt White