Would have mentioned this sooner, but for login problems.
For what it's worth, Volstad and Restayn both show the 503rd's numbers as black and white.
Tigers in Combat I, omitted this detail in the marking descriptions for the Porsche Tigers of the 503rd. However, the proceeding Tiger I's and the following Henschel Tiger II's are described as having black and white numbers . Convention? I don't know.
Osprey's Kingtiger by Jentz and Doyle show 314 "Anneliese" of the 503rd as having red and white numbers.
I'm not sure that the various authors have settled this matter conclusively yet, but I'd put my money on Jentz, which would confirm Culver's Panzercolors. Continued research would yield further opinions, I'm sure.
Looking at large, high contrast images of 334's numbers online and from some of my books, they appear lighter than the balkenkreuz, therefore more likely red.