In the below image you will note that each of the Tracy Boys wear a different colour sash. That's understandable. Scott (who pilots Thunderbird 1) wears a blue sash. Thunderbird 1's 'colour' is blue. That makes sense. As soon as we cross to Virgil, who wears a yellow sash, we find that the colour of the Thunderbird they pilot is not consistent with their sash colour (Thunderbird 2 is green of course). Hence I am wondering if there is any significance to the colour of their sashes and, if so, what is that. I would prefer an in-universe answer, but if there is none (which I fear may be the case), I will happily accept an out-of-universe answer.