Can someone please explain

when someone makes these statements about gear it has obvious tonal issues that stand out and that is why these adjectives are used neutral is when it sounds right to your ears on your speakers in your room when you find a neutral amp in your setup other amplifiers will be bright/dark, warmer/brighter, etc.

How do you not know it’s actually in the recording?

If everything you play has the same type of sound, you know it's not in the recording. Recordings are individual things and they will not all have the same type of sound. Some artists, producers, labels, etc., may have a house sound. However it would be statistically unlikely that every different recording you listen to has the same anomaly.

brighter = shrill sounding, no bass, weak midrange, metal dome bad tweeter. NHT with bryston for example.
warmer = relaxed treble, good midrange, clean bass
darker = mixing a warm sounding amp with a warm sounding speaker. Sonus Faber speakers with a tube or Mcintosh/Classe amp for example.
Neutral - a speaker that does pretty much everything right
Goal is to get either a warmer or neutral sounding speaker and compliment it with an appropriate front end

Compare the sound of a speaker with the sound of live music.
What are the differences?
Then all these descriptors snap into focus:
The speaker is 'brighter' than the performance.
Or 'warmer'
etc

well, ears are like mussels need to be introduced to physical activity, developed and trained:))), but some capabilities are congenital