>> I think NP is normalised power but I am not sure of the significance of the colour.
correct, NP is normalized power. the various colours indicate the Variability Index. You can see this if you hover the mouse pointer over the NP bar.
The VI is the normalized power divided by average power. The VI shows how smooth or evenly paced the power output was during the event.
https://help.trainingpeaks.com/hc/en-us/articles/204071734-Variability-Index
Ride on!