HRV readiness readings is a measurement of how well your body is able to take on a stimulus in a positive way.
You may still be able to train hard when your readiness is moderate (amber) or low (red), without incurring negative consequences right away. However, chronically ignoring your daily HRV readiness and training hard when amber or red, is more likely to result in over-reaching, over-training, and possible injury.
Use your weekly and monthly HRV loads (found to the left of the graph in the web app), as those are more indicative of performance, rather than your daily HRV reading. Be mindful of how much fatigue you have accumulated over the last week, and the last month.
On the flip side, sometimes users will have a high readiness that doesn't match how their body feels. In the case of your body being sore from training, your HRV score will be most accurate, as HRV is much more indicative of readiness to train than delayed onset muscle soreness.
However, if a user encounters a high readiness but feels ill, under the weather, etc., it is safe to use your intuition to hold off on hard training, perhaps decrease the volume and intensity for that day's training session, or possibly take a day off from training.