Ideally you have no yellow or blue, but a nice flat green line.
The green is the minimum latency seen with a 100 second period.
The blue is the average latency seen in a 100 second period.
The yellow is the highest latency within that same 100 second sample.
The odd yellow blip could mean a single ping packet was delayed and is of no real concern.
Long periods with a large increase in blue means most packets are being delayed which will affect latency critical tasks like gaming.
If it's a Virgin connection then a little yellow/blue will always be present, every minute of the day.
It's just the nature of DOCSIS.
With some congested Virgin areas this can take up half the height of the graph, making gaming practically impossible.
Generally FTTC (both VDSL and G.Fast) and full fibre have consistent, flat green lines with any blue/yellow being down to the ISP and backhaul network chosen by you.