There is many ways to do this. But basically, its drawing in the ground color and the drawing itself, then you need to make the layer so it will only color on the ground. Friendly is one way to do it, but you can also do that.
You can also do lock the transparancy for another way to color, so you don't have to keep selecting with a magic wand.
1) they start by drawing with the brush tool on an empty layer 2) magic wand tool with the "contiguous" checkbox checked 3)The flash of red looks like "quickmask mode" (shortcut=Q)turned on for just a moment to check what was selected I guess. and then turned off 4) then Select> inverse 5) then select>modify>contract 6) then it looked like they set the paint brush to color mode and painted on the same layer with the linework
What I saw was, they used the magic wand on the outside so the drawing was not selected. Then they used the hot keys for Select/Inverse , Shft+Ctrl+I Then they started a new layer under the lineart and filled the area with a color, and then they clicked on the Lock in the layer panel so they don't draw outside that area when the area is not selected.
I didn't pay close attention, and I saw that much, so maybe after you use it a few times you will see them do all that.