Mike and Greg are in the bathroom/darkroom developing the print of the touchdown catch. As soon as it develops enough to see the player's feet, Mike turns on the light. In a real darkroom this would ruin the print, as it needs to go into a stop bath after it's developed (into an image).
In the "darkroom" scenes, the photographs that are hanging up to dry face the camera, away from the person processing. This doesn't happen in real life. The photographer needs to see how the images are drying.