I figured out a far, far easier way to make light leaks in Photoshop today. Way easier than previous ways.
Enter Quick Mask mode.
Invert the mask.
With a very large, 0 hard brush (20% flow) brush away the areas you want the leaks in.
Exit quick mask.
Make a curves adjustment layer.
Up the RGB curve a lot. (drag the middle of the line up)
Ditto the red channel.
Do the opposite with green and blue until you like the color.
You can brush in more or reduce the leaks as needed in the mask of the adjustment layer.