Mark Michie put it like this in his letter to the British Library:
I am quite intrigued by the whole subject of colour when it comes to Codex Sinaiticus. According to the photos on the CSP website, the Leipzig pages look so white and the data is consistent, while the British Library pages look significantly darker and quite variable with the data reflecting that variability. It seems very odd to me that one set of pages would be so consistent and the other set so variable. Have there been any causes for this proposed? Has any testing of the parchment been done that might be able to prove why the colour of the British Library leaves vary so much in colour?
This is both
a) from page to page
b) within a page