They could be "light" Wellie hens. I had a "light" Wellie hen (unfortunately, she didn't survive a raccoon strike a few months ago). I picked her as a chick from a bin labeled 'Welsummers' at the feed store (i.e., hatchery quality) because she was uniquely lighter than the others, but then I began to worry whether she was actually a Wellie or maybe even a roo because she was so different in appearance.
It turns out that some Wellie hens, especially hatchery ones, are born with a genetic variation that makes them lighter than others. In my case, my light Wellie also was smaller than her peers. Breeders don't like this coloration. For "regular" chickeners like myself it doesn't matter.
For comparison, my avatar shows my "normal" Wellie hen (the bigger one) and the one with the disfavored "light" genes (little one) as chicks. Both 100% Wellie hens, though. I'm pretty sure my light one also laid lighter-colored eggs than the "normal" Wellie (which laid the classic, dark, terra cotta-colored eggs).
There's a website that talks briefly about "light" Wellies, beginning about halfway down the page:
http://wcna.webs.com/apastandard.htm
I was so concerned about my "light" Wellie possibly not being a Wellie that I started a thread about it a few months ago. Turns out it's just this genetic variation. No need to worry unless you're a serious breeder.