You need both filters. The yellow ones are for B&W printing, and the red ones for Ortho printing. As you note, the top filter that swings away throttles some of the light. I use mine completely shut, and still add a cardboard flap to further reduce the light. The Thomas safelight gets brighter after several minutes use, so wait to do a safelight test. I find mine very bright in a smaller darkroom (about 10 x 17 feet).