Angle and atmospheric composition. Midday sun doesn’t always come at the same angle, because certain countries are closer to the equator. Also, pollution contributes to the color as well. Just look at some photos from China, and you’ll see what I mean. Natural phenomena like sand and ice can change the color too.

Then there’s also the fact that human eye+brain combo messes around with colors a whole lot. What you see (inside your head) is not exactly what’s out there. It’s reasonably close, but your brain does all sorts of tricks with the colors. That’s what some optical illusions are based on, and that’s why photography is so hard.