Nice snake... If you kept the animal it'll be interesting to see what it looks like when it sheds out. That animal may be that color, or it might be colored like that due to pigments picked up from the mud etc that it lives in.

I had just the opposite happen to me a few years ago... Picked up a "typical" muddy cottonmouth, had it in captivity for a presentation animal to show people what they normally look like... It shed out and looked like this.

It seems to be getting more yellow as it ages.

Just for grins... here is a lil western cotton from a few years back. Around here, a lot of the western cottonmouths are very small. There are populations where a 3 footer is a very sizable animal. This animal was just a few centimeters short of record size for western cottonmouths (and a few people on here might get a laugh out of how silly I looked a few years back).