Ian - there are many "isms" in the world of color such as albinism, melanism, etc. It is possible that your bird is an example of xanthochromism defined as "tending toward a yellow color" based on my ancient dictionary. I once banded an Evening Grosbeak which had a xanthic plumage.
From: Ian A. Worley <[log in to unmask]>
To: VTBIRD <[log in to unmask]>
Sent: Wed, May 4, 2016 4:29 pm
Subject: [VTBIRD] Not so brown Thrasher
This morning outside the kitchen window a weak rendition of what seemed
to be that of a Brown Thrasher, with most phrases done doubly, took a
bit of a while to catch my attention.
Went out with the binocs and had good views at the bird as it faced me
from a branch above my head, and no more than 15 or so feet away.
What immediately struck me was that I was looking at a bird with a
yellow face, cheeks, and throat; the yellow bordering on
greenish-yellow. The yellow area of the throat was bordered below an at
the sides by a diffuse necklace of darkish spots. The flanks and belly
were clear, a dull, cream-white expanse. A search for the signature
Brown Thrasher russet (mind you I could not see upper side of the bird)
found a dash of color on the bird's crown, and a wash of russet on the
long, thin tail. Turning its head revealed the striking eye and
I-mean-business bill of a Brown Thrasher.
It stayed in that position for 60-90 seconds, occasionally uttering it's
quiet and limited edition of repeated phrases, and flew away without
revealing any more about itself.
Does anyone have any ideas about the coloration?