Given a set of surrounding colors, how to find a good color template?

This is a map of topics the I cover in my project. The topics are segmented in different stages, and I would like to have your advice on how to color the segments. I check the color wheel to find a suitable color template, but I don’t see one really fit with the yellow sky and blue water. In the end I set them while altogether. I satisfy with this, but since I want to learn more about color theory, if you can recommend a good template that would be nice.

Is the color exactly between two opposite colors always medium gray?

I was trying to find the opposite of a web color (let’s take #917AFA as an example) and an online tool told me that it is #6E8505. Then, I used another online tool to find the color that is exactly in the middle between two arbitrary web colors, typed in these two opposite hexes, and got #808080 (medium gray, exactly). Then I tried doing the whole “experiment” a couple more times and always got #808080, from which it became obvious that it was no coincidence, however it was surprising to me.