More often than not, you won't need to do anything except shampoo and rinse once or twice.  Some oil based paints and varnishes can be stubborn.  If it isn't out after two days, or tomorrow is your wedding day, you can try the following:

With your hair dry, isolate the hair with paint on it from the non affected hair.  Soak a cotton swab in paint thinner and gently stroke the paint away.  Be careful not to get paint thinner in your eyes.

Once the paint is removed, neutralize the thinner with shampoo and water.  Now apply vinegar to the area where the thinner was and comb through the hair for about ten minutes to close down the damaged hair cuticles.  Shampoo - rinse - shampoo -  rinse.

In the event the paint thinner discolored some hair, you will need to go to a salon for a color touch up, or if the area is small, simply cut it away.