It turns out that Thunderbird 5's color is red
I come to this conclusion on the basis of this image:
This image is taken from the film Thunderbirds Are Go! at the very beginning (I don't recall the exact point in time, but it was within the first 20 seconds at the beginning of the actual movie).
You will observe five colors, each representing a different vehicle:
- Blue (Thunderbird 1)
- Green (Thunderbird 2)
- Orange (Thunderbird 3)*
- Yellow (Thunderbird 4)
The only remaining color is red. Since there are five Thunderbirds, and the last color is not pink (which would indicate FAB 1), it is reasonable to assume this must be assigned to Thunderbird 5. Hence Thunderbird 5's 'color' must be red!
This is not such a ridiculous idea, as if you look closely at the image in the original question, you will note that there is red present on the Thunderbird just below the '5' near the docking port.
*There is some confusion about Thunderbird 3's color. I always thought it was red and many pictures suggest it is indeed red (take a look at the Thunderbird 3 page on the Thunderbirds Wiki for examples). However, according to The Complete Book of Thunderbirds 40th Edition (Bentley, 2006), at the top of page 47 (the page on Thunderbird 3) it clearly states (bold is my emphasis):
Thunderbird 3 is International Rescue's massive orange space rocket.
This is what I base my argument on.