The mix node when the direct and indirect passes where combined was set to add. The mix node was set to multiply when the combined indirect and direct was mixed with diffuse color pass. Why is was first set of nodes added and the second set multiplied? What is the difference? Time on video is 1:37.
I'm not a compositing expert but if I remember correctly.......
multiply darkens the image with the color's brightness 'value' that you multiply with (I believe the color saturation is not considered and only the brightness value are taken into account if I remember correctly). (think multiplication of numbers between 0 and 1. 0 being black and 1 being white)
The add tends to lighten the value of the color I believe by adding the value (think addition)
I really like this YouTube video. It will explain it better than me but I believe this closely applies to all color mixing in blender (to my knowledge):
The formula for combining render passes to recreate the render 100% perfectly is very specific. I don't know the exact math behind the formula, but it's demonstrated in the Blender Docs.
Again this is only formulaic for combining passes to exactly match the completed render. Apart from this I use blending modes artistically; not according to a strict formula. In which case Shawn's video is a great resource for understanding what each blending mode does.