Dying leather is a long, smelly, dangerous process - and that is under factory conditions. Dying "finished" leather is even harder, nastier and has a STONG PROBABILITY it won't work. Dying often leaves residue that comes off on your clothing for years, and years, and years.

Leather is tough to penetrate. Heavy metals and urea (yep...piss) are needed to get the dye to transfer into the material. Huge amounts of water are needed to dye/rinse/dye/rinse. If the leather is finished and sealed it is almost impossible to get it to take another colour.

If the chair is in great shape, have it reappolstered in a leather you prefer.