Color Vision

Cariquinez Bridge - 8 x 10" oil on linen - plein air painting by Linda Schweitzer

For years, I have known that, in theory, the larger the pupils of your eyes, the more color you can see. Because of this, I sometimes wear a visor, even when painting in the studio. But I say "sometimes" because I've never really noticed if it made any difference.
Well, this is the painting from my last post. It was done plein air, meaning outdoors, from life. As I was walking towards the water, I saw that it was a lifeless blue-gray color, and I was a little disappointed. But I set up the easel, and the umbrella. By the time I was ready to paint, the water appeared a beautiful aqua! I thought, "Good, the light has changed!" And I proceeded to paint, but could hardly believe the color, and kept checking my mixtures against what I saw.
When I finished, I packed up my stuff and started back up the hill. Looking back at the water, it was that blue-gray again!
Now, if you look at the photo on my last post, you can see I was in a pretty deep shade under that umbrella. It is a Best-Brella, which is silver on the outside and black on the inside. And I was wearing a broad brimmed hat. With all that shade, my pupils were probably quite wide even though the sun was full blast. I know some artists say a white umbrella is enough, because it gives a filtered light which is more gentle, and your colors will be more accurate. What do you think?