Most of the time, going over a darker colour isn't a priming problem — it's a coverage problem. A modern matt emulsion will eventually bury anything underneath it, but if you skip the prep you'll be slapping on four or five coats of expensive topcoat and still seeing ghosts of the old shade.
The smart move depends on the gap. Going from a mid-tone to a slightly lighter shade? Two solid coats of your topcoat will sort it, no primer needed. Going from something genuinely dark — say Farrow & Ball Hague Blue or Studio Green — to a pale finish like Pointing or Wevet? Get a coat of primer or undercoat down first.
Use a grey primer rather than brilliant white when you're lightening a wall. It sounds backwards, but a neutral grey base (most brands offer a tinted primer, and Dulux do a grey trade undercoat) evens out the surface and lets your topcoat reach its true colour in fewer coats. Brilliant white underneath a deep shade can actually fight you. For tricky jobs — old gloss, stains bleeding through, or a wall that's been wiped down with something dodgy — reach for Zinsser BIN or Cover Stain to lock everything off before your emulsion goes on.
The "but what about" question I always get: *can't I just use more topcoat instead of primer?* You can, and on a small wall it's often cheaper than buying a separate tin. But if you're covering a strong colour over any real area, two coats of primer plus two of topcoat works out faster and more reliable than four coats of premium emulsion — and a lot kinder on the wallet when you're using Little Greene or Mylands.
Practical bit: always let each coat dry properly before judging coverage — wet paint looks patchy and tricks you into a needless extra coat. Roll in good light, stand back, and only recoat what genuinely needs it.