One way to achieve year-round color in warm season turfgrass--such as Bermuda, zoysia or St. Augustine--is to overseed your lawn in fall with a cool-season grass such as Kentucky bluegrass or ryegrass. Grass seed is planted in established lawns by creating shallow furrows in the turf so that seeds can come in contact with the soil to germinate.
Mow your existing grass to its shortest healthy length. For many grass varieties this is 1 inch long. Rake up grass debris and bag it for disposal.
Rent a vertical mowing machine from a lawn-and-garden store. Vertical mowing machines operate similar to lawn mowers in that they are started with a pull rope and pushed over a lawn in a grid pattern. Vertical mowing machines are designed to gouge strips into your lawn to remove thatch or create planting furrows for new grass.
Read the instructions of the vertical mower so that you are familiar with safety precautions and how to change the machine's settings. Operation of vertical mowers varies among manufacturers.
Set the blades of your mower so that they cut a 1/8- to ¼ inch-deep furrow in your turf.
Mark any obstacles such as sprinkler heads or rocks so that you can avoid them.
Push the vertical mower so that it always cuts grooves into your lawn at a 90-degree angle to the lawn. Slit-seed the lawn in rows that run the opposite direction from the lawn's slope. For example, if your lawn has a slope that runs north to south, your furrows should run east to west. This will make plantings on hillsides more efficient, and will prevent erosion.