All depends of the film type the pic was taken with. I would say the yellow but the red one is very likely too. HOwever the black outline is more characteristic for the yellow codes.
Jan. while looking fro more info of the "13" I have found the pic. The caption says the "Red 1" and I would say that's it. The small triangle with the fuel octane info is almost unseen. If the shot would be taken with the film that shows the yellow colour as the dark one the trinagle was of the same dark tone the digit is. The pic seems to be an evidence that the squadron used the red numbers with the black outline in 1939/1940 .
That's true Terry. However the fuel marking should be dark too if the "1" would be of the yellow.
Here is a shot I found a while ago. The "12" doesn't seem to be of yellow comparing to the fuel triangle that is very light.. but of course I might be wrong.
It's also interesting to note, even though that the Yellow 6 and Yellow 9 are within the same Staffel, they're both in a very different style from each other....the 6 being much more rounded in shape, the 9 much more square....