The codes had been shown for many years as red, until the colour pic emerged a few years ago. There's been some discussion on this here and there, and I think here on the forum too. The 'red' was possibly deduced from study of tonal ranges of B&W prints, and the fact that other units often used white-outlined, red codes.
However, that colour pic disproves the red, and the Airfix 1/72nd scale Spit Mk.IX released in 2011 includes the blue code letter decals.
I can't be certain, but I think the 'lettering' on the rudder trim tab of 'ZX - 1' is a repeat of the serial number, as I recall seeing that stated somewhere just a couple or three years ago.
The codes on 'WFD' (Gp. Cpt. Dickinson, from memory), have always been shown as red, a common colour on Italy and Mid East based fighters. I've seen a colour image also showing red, and although I'm not certain, the B&W pic might be from a colour original, judging by the tonal reproduction.