For old stain try using a paint remover (from a hardware store). Test a small area first. This is a fairly harsh chemical designed to turn paint to mush so it can be removed from furniture for refinishing etc.
Scrape and remove (using a spoon) as much of the paint as possible.
If the paint remover is working apply more as needed. Be careful not scrub, this will force the paint deep into fibers.
If the stain is still there, cover with glycerin and let sit for several hours in an attempt to loosen the stain.
Then treat as a fresh stain and start by sponging out as much of the stain as possible before applying a laundry pre-treat – launder in warm water. Do not put in the dryer. Inspect the stain to see if you have to try again to remove it. Drying in the dryer will set the stain.
If you are fond of these pants try going to a professional cleaner. Point out the stain and ask if they think they can remove it; they likely can with better results than trying at home.
If the paint is oil based the pre-treatment should be a solvent designed for oil based paint (paint thinner).
If all that doesn’t work, dip your jeans into a pail of green dye and at least you’ll have a consistent color!