Hello olele and every others users of zygrib:
I also use zygrib for the weather forecast in my country, and is very useful tool for me. Like you I have seen that colors of range and scale tend to confuse, I know that our friends from zygrib are already working on that.
A "trick" that I used in this cases, is disable the option Smooth colors in Data --> Options (Ctrl+F), and with that I can better define the ranges, but the map doesn´t look so cute
I hope that this help you for now.
Zygrib Team, thank you for keep working in this software