One might suggest that the explanation is that in this case you call “blue” what is in common between the two colours, and that, if you called “strain” what was in common between the two experiences of strain, it would have been wrong to say, “I called them both ‘strain’ because they had a certain similarity”, but that you would have had to say, “I used the word ‘strain’ in both cases because there is a strain present in both.”