generally you would only do this to lower the volume of peroxide. EG freshening up colour through mid lengths and ends without creating any more lift which would cause the colour to fade even faster.
420 is majirouge, not majirel. Majirouge needs a small element of lift to get the tones vibrant. Too much kills the blue pigments in the 420 though, so maintain really good condition, as porosity will really fade it badly.
Aw wish I could come and work with you for a few weeks I want to know EVERYTHING about colour lol xx