My first thought when I saw him - still a young boy, and young laced stock go through dramatic changes as they mature.

I don't know if Db is involved in your bird. Mh - Mahogany might do similar strange things?

If you start getting bleached out (silvery) chick downs, then you can start suspecting Db (or Di). Co (particularly with Ml -eumelanisers) should add eumelanin to the chick down, particularly the back. And Co/Co tend to be darker downed than Co/co+. So the chick down might give some clues.

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As to Db - Dark Brown. It is very similar to Co in that you can have eumelanisation of neck hackles - just add heaps of eumelanisers.
Eg, this following rooster went through some similar changes as a youngster (particularly on the breast):




One of the most tested roosters I possessed, so I know the following as accurate:

His genotype:
e+/e+, Bl/bl+, Db/db+, co+/?, Lav+/lav, Mo+/mo, Pg/? plus eumelanisers.

p.s. -his parents were Black Mottled father and solid Blue mother (dark blue). So Db can hide in solid eumelanin birds, including with het. E/e+ or ER/e+.

When I crossed a closely inbred sister (with e+/e+ and Db like chick down) back to this blue boy, I got some unusual results in the sons (hens were drab brown though - not pretty).

Eg:


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The following boy is a much later generation, distantly related, with wheaten in there too:
As a youngster:

Older...

-definitely co+/co+ (cream chick down, not buff), but don't know about Db, & was a red enhancer introduced too. Something was putting partial phaeomelanin on the breast.

p.s. - I never once got a solid phaeomelanin neck in these d'Uccles segregating Db. They went from columbian striped right through nearly solid eumelanin (including ER/e+ or E/e+ ones). Always eumelanisers in the mix.