I use gels on my indoor lights all the time. I always try to keep them from touching the lamps (bulbs.) Usually I just slip or clip them into place. I'd be careful around halogen though... keeping a few inches between the gel and the lamp with room for air circulation. With LEDs, you can put the gel right next to the lamp. On my recessed incandescent lights, I just tape the gel across the fixture edge. I use mostly Rosco stage lighting gels bought in 24" x 20" sheets. They last forever and allow you to quickly change color without having the expense of colored bulbs.