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.

Bob Yunick
Schenectady, NY
-----Original Message-----
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?