To dye any type of fabric, whether it be leather, microsuede or a poly blend, the fabric must be submerged in the dye. Spray-on or paint-on dyes do exist, but this is not really dying. Dying requires the use of heat and a lot of rinsing to remove excess dye from the product. Therefore, it is not possible dye a couch because submerging a couch in a vat of dye would result in damage to the frame, foam and other components.
There are several products available that can be painted or sprayed onto the couch's surface. Ideally, the cushions should be removed and the foam inside the cushions should be removed. There are alternative ways to recolor existing fabric on a sofa, such as using fabric paint. One such product, Simply Spray, uses fabric paint to change upholstery coloring.
A sofa should be thoroughly cleaned and allowed to dry prior to application of any dye or fabric paint. Whenever using a recoloring product, always follow the manufacturer's instructions. Before beginning any couch recoloring project, be sure to protect the surrounding areas and furniture with large drop cloths.
Though it is possible to recolor existing fabric on a couch, it should be noted that the dye or fabric paint may rub off on other items. This could include throws, pillows, clothing and even carpeting or drapes. Recoloring fabrics like microfiber should not be attempted, as this can result in the tiny fibers sticking together, which destroys the look and feel of the microfiber fabric.
Instead of recoloring a couch, consider purchasing a slipcover in the desired color. You could also buy a slipcover made from natural fibers and dye it the desired shade. A slipcover can give the sofa a new look with less mess and less hassle.