White kings are genetically unable to store carotine (same stuff that give carrots their color) in their flesh, even tho they eat the same diet as red fleshed kings. Their are a number of Fraser River tributaries noted for their prevalence of white kings, like the Harrison River, so that is why they used to be common in catches out in the Straits. The Skagit River does get a few white kings as well, but they make up a small portion of the run.
I did catch a white fleshed, bright winter steelhead in the Toutle many years ago, before the St. Helen's blast. Hadn't heard of white coho, but there is no reason why such an aberation shouldn't show up in any of the salmon or trout species from time to time. Consider that it is probably just a genetic fluke that gives most salmon the carotine storing characteristic that gives their flesh such an appealing color. Ask anyone in marketing, red sells better. So regardless of taste, it's no surprise that most consumers prefer the red kings over the white. I've eaten a few white kings and honestly could not detect a difference. And when it comes to salmon as table faire, I'm about as narrow minded and snobbish as they come. i.e., I fish for steelhead, but I release them and pay bundles for Copper River king salmon because it tastes better to me.
Regards,
Salmo g.