Could you have just changed the black in the colour ramp to red?
Yes... but doing it that way would not have included the random object shading provided by the other nodes... so you'd have to add that on top of the result of the color ramp if you wanted that in there.
It's a case of there being multiple ways of doing things :D
The way he has it set up.. .he's only affecting the spots, not the eventual color of the mushroom.