If you want your horse to have a show coat in April, I would body clip mid to end of January, *before* the spring coat starts to grow. The idea is to clip the winter coat short instead of shedding it out, and that the spring coat growth shows up immediately instead of gradually.
If you clip too late and clip the spring coat coming in, you'll end up with a horses with the odd, lighter, matte color.
If you do clip in January, you'll also have to be very diligent about blanketing.
How much of a color change you get depends on what color the horse is, what stage their coat growth is and when you clip.
PS - putting a horse under lights work by altering the Circadian cycle; the length of light and dark periods during a day. The horse has to be in a stall, obviously, and that light has to be pretty bright and in a certain spectrum for it to work. Most people put the lights on timers to mimic the day/night cycle.