Each tire has a cycle. Blue is cold, and slippery. As tires warm up they get green, then yellow, then orange. The best traction and stability seems to happen when tires are yellow, but even orange tires are good. When they turn red, they are even worse than when they are blue.
However, if you compare yellow T2 tires and T5 tires and yellow T5 tires, the T5s are still better, even though the T2s are as good as they can be.
Also, if you are finishing the race on yellow tires, you will lose time at the beginning because the tires are warming up slowly. You are also ending the race with tires that are not worn out yet. Try going to one level softer tires so that you finish the race on orange tires.
So really, the best strategy is to use the softest (highest T number) tire that will last through the whole race without pitting.
If the race is too long for one set of tires, use the softest tire that will last to the middle of the race, and pit once, after the middle lap.