that reflects essentially what I am seeing; sometimes the example in the 2nd pic has a yet lighter hackle with the dark overlay.
I kept your comments on shades of grey and APA response to variety, as those are questions I am interested in.<edited>
Gray can go from nearly black-gray (slate like a black board) to nearly white -- hence the saying 'it's all shades of gray" - when things get indefinite.
I wonder if the APA would allow the range of crests that we are seeing.
<edited>
ETA - on the CL Club's website we do have a photo of a prize winning CL 'Lillian' - and she appears to my eye to have the darker crest. At least that is one benchmark.
ETA again - here is a photo of a winning UK Pullet in 2014:
With regards to "Jillian", it is helpful to read that matches your interpretation of a dark crest. Although I think the photo seems a little dark, and therefore the hackles don't scream cream (?), I do see that particular crest and hackles as matching; I don't see them as overly melanized. "Jillian" can also be found on the home page of Jill Rees' website as well as other examples of her hens that appear substantially brighter in the crest and hackles, as well as darker crests that seem to include darker/melanized hackles. Maybe there is another way to describe the increased dark detailing on the hackles...
http://www.jillscreamlegbars.co.uk
As a side note, in the Black Copper Marans, there is a similar transitional color area at the top of the hen's head that judges look at. I don't know how heavily it gets docked? While the judges are looking for the copper to extend through the hackles well into the head area, there is often more of a black cap that extends from the top of the head down into where the copper should be found. Maybe there is a way to define this trait that seems similar to the dark crests of the Cream Legbar heads, which then layers over and obscures the cream in the hackles...
If these nuances are not important to the breeding programs of the Cream Legbars, then that's one less thing I need to consider...