SInce I can't count of flash to work for me all the time because of the fish's attack vector or position , poor vision, particulate diffusion, weeds in the way while the fish attacks, etc., I did the scuba diver thing in every condition possible over the years to see what the sky color might be.

Here are my observations:

Most of the time visibility is poor unless water clarity is PRIMO. Much of the sunlight light other than at sun noon and two hours each side is reflected back off the surface. The sky 'color' was not important, as variables made that too big a leap to figure out exactly. What did matter was the amount and type of cloud cover, and the sun's position during the day creating a general background. Background is VERY important. On a cloudy day, generally speaking, the background is from white to grey to dark grey. A dark color or black will show against that background very nicely. On a sunny day, depending on particulate and sun height, position on the axis, etc., the background will be blue to violet, against which a light colored bait shows very nicely. Florescent colors seem more intense. White shines like a beacon.

At night an interesting thing happens. The surface takes on a glow, and appears lighter by a considerable amount than looking forward or down. There is a name for this which escapes me at 1 AM, but fact is it's there. Black is an excellent choice at night. So is white, but that is harder to pick out. Add moonlight, magnify what I just said.

My 2 and 1/4 cents.

What if the UV spectrum was a factor?

The UV issue....

I'm under the impression that animals (fish included) have certain properties in their unique eyes that respond differently to light and color than we do. I appreciate your curiosity underwater but would probably give science a bit more weight as far as analyzing how a certain fish sees color and light. Their rods and cones are unique for their underwater enviroment so obviously conclusions based on what "we" see under water with our human eyes would not be conclusive. Facinating none-the-less. Unfortunately, I can't think of any articles referring to this difference from a scientific point of view. I know I've read some before though. Just a crappy "human" memory I guess!

You are correct. Muskies have both good color receptors (cones) and very sensitive vision for low light and night (rods). The cone receptors extend on a 'clock' basis, same time every day, and the rods take over basically speaking in the same fashion.

General perception is that as the light under the surface builds, the color vision is ready, and as it drops, black and white takes over. The factors effecting the fish's vision are the obvious, like water clarity, wave action, types of particulate, and visual acuity.

Interestingly, visual acuity in pike and most other freshwater fish is no better than 4' (Schwassman, 1974) (Muntz, 1974). The conclusions in most vision studies on acuity showing about 4' to 5' for most fish seem to indicate that the need for better acuity isn't  critical because better acuity serves no purpose in the underwater environment.

It is clear that some fish do have ability to see in the UV spectrum. UV is absorbed rapidly, but is available to most freshwater environments. I haven't been able to locate anything in the literature that indicates how fish actually use  UV vision.