it'll wear off the seats long before you ever take the covers off and wash them. you'd be better off taking the covers to an upolstery shop and having them redone. plus you wont have paint flakes all over your ass and back every time you get in the car.
the spray dyes generally only work on plastic or vinyl, as long as its a surface that doesnt get touched constantly.
SEM makes some stuff that works pretty well, called color coat. probably the best performing stuff ive used to date.
oh make sure whatever you're painting is clean. and i mean CLEAN! wash the part with soap and water, then wipe it down with denatured alcohol several times. if its shiny it wont stick there for very long. if you think its clean, wipe it down with alcohol one more time anyway. if its possible remove the part from the car when you spray it. it'll look better and be a lot easier to clean.