Stages Flight uses color-coded zones based on FTP (functional threshold power) to help guide you through intensity levels based on color. The gauges within the class will change colors based on where you are riding in relation to the FTP set in your profile. 

Zone% of FTP
Zone 1
Zone 256-75%
Zone 376-90%
Zone 491-105%
Zone 5106-120%
Zone 6121-150%
Zone 7151+%