Once a week I photograph adoptable animals at the local humane society. We always shoot against a white seamless background. To mix things up, typically we throw a gelled speedlite on the background. Sometimes I get a bad colored halo around the critter. I have the pup as far away from the background as I can, typically 4-5 feet, as we are working in tight spaces. The speedlite has a Honl gobo on it to prevent spill on the subject. The speedlite on the background is about 4 feet from the background and the power is turned way down (PW AC3). I don't always get the halo and am baffled as to why it occurs only sometimes. Any ideas would be greatly appreciated.
The first issue is: shallow DOF. Basically everything not in focus gets blurred, and if you check your colors, the halo is the average of the dog fur color and the background color...
The second issue could be your image handling. If you resize photos of animals with fur then you are basically blurring those fur segments, and you are introducing color shifts during the resizing (in Lab it is less noticeable).
And, let's not forget: you could try moving your flash off-camera if it is currently on it.