I was painting and I got some black color poster color on one of my white jeans. The stain is pretty visible and after washing the jeans, it was still there. Getting rid of the jeans is not an option. How to Remove Black Color Stains from White Jeans
It takes "elbow grease": a lot of gentle rubbing with detergent and a toothbrush, assuming the color is water based. Scrub with the grain of the material and at 90 degrees, to remove insoluble particles. If this is solvent based, you'll need an organic solvent such as naphtha or bug & tar remover with xylene or toluene. Be careful, those are flammable and a bit toxic!
Too late for this time; but, getting the pants to a professional cleaner ASAP is the best first option.
If you've let the pants dry before you have completely removed the stain, there's a good chance you will never get the paint out.
Before you start, it's important to find out what the material is that you want to remove. Read the label and note what the stuff is made of and the manufacturer's coordinates. Contact the manufacturer for what to use and how to treat your pants. Most of the time, they can advise you what to do and how to do it. No? Then do as much research on the Web as you can. Don't accept the first answers but compare answers for the ones that seem to make sense and are in agreement with each other.
If you've already washed the pants and the stain reappears in a different colour stain (brown, say) appears before drying, drying will set (make it permanent) the stain remaining. It's probably the paint "vehicle" (the liquid stuff that the pigment has been mixed in) causing the stain. If the stain is the same colour, continue with a second treatment, and maybe a third. Rinse liberally with water to remove particle rather than scrubbing them into the fabric.
Scrubbing one part of your pants will damage the area so don't rub hard.
Only dry the pants if the stain is not visible when wet.
If (when) your efforts to remove the stain are not successful, permanent white laundry markers are available that might work to cover the remaining stain. Apply the white dye by daubing the pen rather than rubbing back-and-forth. You want the white to sink into the fibres thoroughly. Let the marker dry completely for maximum effect.
If you were wearing black pants, Murphy's Law states that you would have been trying to remove white poster colour instead. Sometimes, you just can't win.