Tony,
Back in the office and done some checking: The chassis numbering of the Ford-manufactured No.4 trailers was a straight number sequence, so the number you gave - 1712 - does conform to a Ford No.4 chassis number, while 'JT12' conforms to the Ford numbering system for their No.3 and 3A trailer manufacture. Sorry to have confused things earlier.
Anyway, checked the number '1712' and found it in my trailer notes. It had the ARN
166778. It was disposed of in the immediate post-WW2 period, so you are very lucky to have found it in such great condition.
As to paint colour, it would have originally been delivered in the standard Khaki Green No.3 (second version), so to be authentic, that's the colour to use, along with the ARN in white numerals, 3 to 3.5 inches high, with the width of the lines making up the numerals all 1/2 inch wide. But as I said earlier, it's your trailer & your choice. We await news (at completion) of your final choice.
Regards
Mike C