painted details on leather.

[M. Demetrius]

Romans of the first and second century could decorate their leather . . . with details painted as in the republic.

Seems like they wouldn't have forgotten. So if they didn't it would have been fashion choice rather than inability. Beeswax based paints are a little flexible, and dyes are very flexible. They had both in both eras.
(Hah! The first time I typed it that it came out "both ears". :razz: )