do you know whether he uses amino dyes (mixol makes some) . Ive often used Feibing leather dyes (They are essentially amino based dyes) They change color if you add water v alcohol or acetone.
You can get blues and other neat tints. If you paint em up and experiment with the times then you can wipe off the residue and lightly sand the surface, the dye will stay with more absorbent rings .
I guess you could use a vacuum system but that might send the dye deeper.