Try a 3/4 liquid dish soap (made for hand-washing dishes, not the kind that goes in your dishwasher) and 1/4 water mixture. Soak a sponge in this and blot quickly for as long as it takes to remove the paint.
Acetone, Alcohol or Mineral Spirits
Before using any of these compounds, test them in an inconspicuous area of the couch such as the inside hem of the skirt. If they stain or bleach the area, don't use them. If the test works out well, blot the paint-stained area rapidly with a sponge soaked in the compound.
This works well on leather couches, but may leave the area darker than the surrounding material. Work oil soap (such as Murphy's) into the area of paint with a toothbrush and wipe away with an old towel.
If the Paint Has Dried
If the latex paint has been allowed to dry on your couch, none of these methods may successfully remove the paint. In this case you could reverse the cushions so that the stain does not show, re-upholster the couch, or cover the couch with a slipcover.