o.O thats not an answer, and the scheme would require at least two colors chemically consistent  enough to have the same x-ray profile to have a solid background with text overlays in a different color not show any differentiation between the two in terms of xray opacity.  

 https://hackaday.com/2021/03/06/x-ray-defeats-letterlocking-unfolds-and-reads-letter-sealed-since-1697/