We all know what a fantastic modeller you are, so I'll be different this time and congratulate you on the photography.
On the subject of livery, we probably have the same books so I'm afraid I can't add anything. I wonder though where the theory of the lettering originally being black actually comes from? Slinn does not state the origin of this theory. Indeed, he makes a point of saying that no trace has been found in official documents to say which colour was used, and that the difference between black and red cannot be seen in photos from the time. So where does the theory then come from?
On a related issue. I assume it is a scientifically proven point that the photographic emulsions of the time don't allow us to differentiate between red and black? I ask because when I look at the photo of the Mica on page 94 in Slinn (original version), the lettering seems obviously red to me, although it is "believed to be in black".
The plot thickens when I look at the photo on page 97. Here, some of the lettering is much darker than the rest. In fact, the darker letters almost seem to have been touched up (note especially the "G." bottom right, compared to the "W.R."). If indeed some of these early photos were touched up, could that have given rise to the theory of black lettering?
What do I know. But conspirancy theories are fun!