If Lamm is dark then the opposite of dark in this case is not light, but lean. I consider Lamm as having more dense or more complete low-mid to low frequency tone and harmonics which is how I interpret comments about dark.
The systems that I have heard with Lamm gear have tended to be very open, transparent, and resolving, also tonally balanced. A couple of visitors recently told me that they did not think female voice had enough weight, which I interpreted as a bit tilted up in terms of tonal balance. I adjusted the cartridge setting and it sounded better, so I thanked them for their comments. No one mentioned the word "dark".
I think of dark as either closed in/less open, or as lacking information. I also think that it might be tilted toward the low frequencies in terms of tonal balance. I do not like the term "dark" because the meaning is not clear to me. No one has described my new system as "dark", and I would certainly not ever think that a cartridge like the Colibri would "lighten it up" or add some leanness.
This is where language utterly fails, IMO.