@Chiara I moved your question here because, even though you are asking about T-Score in Today’s Plan, that metric is very similar to the TSS Score in Training Peaks (which I explain in the video above).
If you hover your mouse over the metrics in Today’s Plan, you will see a brief explanation of what they mean:
So the T Score is a composite number that takes into account the duration and intensity of a workout to arrive at a single estimate of the overall training load and physiological stress created by that training session. Nevertheless, it has its limitations, as explained in the video above.
When it’s red, it means it was not as close to the estimated TScore for that session (either up or down). When it’s orange, it means it was close but not exactly there.
I never worry about these, as often times the difference between planned and completed scores have to do with rest intervals - which are not meaningful in a high intensity workout.
More on T-Score and other Today’s Plan metrics: