TL;DR - add an option to change the color of each power zone when selecting the zone values.
Please consider changing the default colors of the power zones. Or provide an option to manually change it.
The Aerobic and Tempo zones are both blue and look very similar depending on the lighting. I do most of my rides in the endurane (aerobic) zone, and I need to be careful not to go out of it. Different shades of blue are not enough for me do differentiate them. Consider purple as it is at the end of the spectrum. Or just plain white - there is nothing wrong with white esp. for that zone. You have 64 colors now to play with, please don’t use so close colors. I agree with the ‘gradient’ idea but it isn’t that useful when they overlap that much.
Aerobic and Anaerobic zones are also similar. The difference there is bigger so I am fine with orange, but I think darker red for anaerobic will provide higher constrast.
Gray for recovery is fine, and the difference between green and yellow is noticeable too.