Can someone please explain

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 

ears look more like shrimp than mussels ...

the shrimp is actually big mussel itself:)))