Brush dirt and debris from the sofa. Remove the cushions from the couch, as well as any removable covers, and beat them outside. Use a crevice or brush attachment on the vacuum to get rid of as much dirt and dust as you can from the rest of the sofa.
Wash the fabric of the sofa and the cushions using a furniture cleaning machine (for best results) or wipe down with a damp rag and upholstery cleaning soap. Let dry fully before you paint.
Test the spray paint on a portion of the sofa that's not easily noticed, such as the back or bottom. Spray a little paint there and monitor it to make sure it doesn't react with the existing dyes or chemical makeup of the sofa's fibers in a way you don't like.
Apply a coat of primer color to the sofa; this will give you an even foundation that won't interfere with your color. Unroll folded areas of cloth (such as those in the skirts of many sofas) to make sure they get sprayed. Use a paintbrush to work some paint into nooks and crannies in the cushions, folded portions of cloth, or areas with stitching and buttons. Let dry.
Coat the cushions with primer paint. If the cushion covers can be removed from the upholstery within, do these before you paint and paint only the covers.
Turn the sofa upside-down to primer spray any places you might have missed while the sofa was upright, such as the folded areas of cloth that wrap around the bottom. Let dry.
Spray the sofa with a first coat of colored paint. Use the same process you did with the primer coat to cover all visible areas of the sofa.
Add a second coat of colored paint. Let dry.
Reassemble the couch and cushions.