Photo 18: LBBG adults, April 16 2001, Nachtegalenkeet Maasvlakte, the Netherlands.
An image showing the difference in grey tones between two LBBG in
For comparison an exact age would help. The grey and brown tones between fresh feathers and old feathers can be compared. remember that an individual actively moulting coverts may look darker than a bird that has completed it's moult.
For instance, see the new central tertial in the bird at the background: relatively pale grey, not unlike the new feathers of the bird in the foreground!
Note the size of the primary tips.
Also, note the red spot at the gonydeal angle, extended to the upper mandible in the far bird.
Note p10 of the bird in the foreground: the outer tip and the sub-terminal band are broken.
These two birds may both be females (note the facial expression).
To find more information about the different shades of grey and brown mahogany hue in various LBBG colonies throughout Europe, check the article written by Barth (from the 60's).