Yes, now I see that my method had sense only in my head... However, you can do the opposite; you can change Layer colors in drawing a, save it... After you open drawing b witch contains a as xref, you can set sysvar "VISRETAIN" to 0 save and reopen drawing b... Now colors from drawing a matches colors in drawing b, and if you then change them from b, again save, and again reopen b, changes are discarded and again colors from drawing a matches colors in drawing b... If you want to keep changes made during session b, you'll have to set "VISRETAIN" for drawing b to 1, save b, reopen b, and changes are preserved (colors a don't match colors b)...
M.R.