Lean is a coloration in this case imposed by the cartridge.
Lean is not a colouration imposed by the Lyra cartridge.
I understand the adjective 'lean' as a lack or absence.
- Lean meat lacks fat, lean years lack bounty.
- A synonym might be 'sparse'.
- An antonym might be 'full' or 'rich'.
To me, then, a lean sounding cartridge is lacking some characteristic that is presumably a positive or desirable. Better to describe that than simply saying "X sounds lean".
Whether to say "a cartridge sounds lean" is to describe a coloration ... I don't know. Presumably "a coloration" is some added characteristic to someone's idea of neutral or uncolored. Neutral only makes sense when relative to something else -- there is no universal neutral.
It's all a big mess, yet we think we are communicating.
The more general a description, the less informative it is.