I am red-green colorblind. So certain shades of colors like that I literally can’t see. Blue and purple are issues, and certain shades of yellow and green.

In the image above, I cannot really see a difference in the extremes of the legend. The gradient is definitely not helping. I can see the difference when put near each other. Like Mississippi and New Mexico are clearly on opposite sides, but I would not be able to tell you which direction either leaned because I can’t see what the legend is showing. Likewise most of the states mean nothing to me since they’re part of the gradient itself going towards colors I can’t distinguish a direction from. Without the numbers, this map would mean nothing to me.

Simply put, pick colors on opposite sides of the color wheel when trying to show differences like this with gradients and you’re more likely to to okay. Don’t pick colors that are next to each other. TRhis might as well have been a blue/purple gradient with an extra z-dimension for time or some crap,

It’s not gray, almost no one actually sees in greyscale, despite the jokes. It’s always just certain shades the eyes can’t distinguish differences from.

A good example for other common colors, is peanut butter looks like a shade of like greenish tan or maybe dark khaki, not brown as most people describe it to me.

A decent resource to explain visually for those that can actually see the full range of color: https://enchroma.com/blogs/beyond-color/how-color-blind-see