Hmmmmmm....Sam's tail feathers are a different shade to his normal body feathers. His normal body feathers are yellow, BUT two of his tail feathers are browny-yellow. I've also realised that some of his feathers on his wings are different shades of yellow too. Like, most of his feather is yellow, BUT, the tips are a darker shade.
Have you got pictures of what you've described...and are the "darker, brown, colouration" only on the edges of the tail feathers....?
If it's brown markings it could be signs of cinnamon ...while, if such "dark markings" are on the edge of feathers, it could indicate "spangle"....but for the latter gene it would have been more evident from such (similar) markings on wings...but lets wait for your photo...
...I'm sure the TB experts will have the answer for you quick-quick!
Hmmmm....interesting. At first I thought he was a lutino. Her eyes are like a REAAALLLY REAAALLLY REALLLLY dark reddishy colour, possibly black...but she is nearly one year old, so possibly it her eye shade is getting lighter.