I wouldn't try this on a nice newer car but if you'r talking about something that doesn't have much market value, and on the non-seating surfaces, I would say it's a viable project. The "dyes" on the market, Leatherique, ColorPlus and SEM Sure Coat, are not penetrating dyes, they are surface coatings with very good coverage. It sounds terrible but they are essentially paint (polymeric, elasomeric) that remains flexible. They also wear very well. On a non-wear surface you can put them on quite thin, not lose the grain of the leather, and still get good coverage and durability. I think it's a question of the market value of the car and how much work you're willing to put into it.