Terry - I think you’re referring to the NP column of data, within ZwiftPower, and how sometimes there is a red, orange, green, yellow bar going part-way across that rectangular bar containing your np number.
That coloured bit refers to the variability index… how much your power varied during the course of the race. This page has some info about what a variability index is.
My basic understanding is that you have red in that area when your power level fluctuated widely … so you had a very uneven ride that day. Then the green, yellow, green are various levels taking you down to no colour at all, which is indicating that your variability index is close to zero… so you were doing nearly the same power level for your entire ride.
It probably goes in this order… though this is me going by memory and also guessing:
Red - widely variable ride… power level all over the place
Orange - varied, but not as bad as red
Yellow, - varied, but not as bad as orange
Green - pretty tight… close to a long consistent effort
no colour at all - very tight… nearly the same watts the whole ride
that’s my understanding anyway!