MH [Metal Halide] is a blue spectrum light used in the vegative stages of growing.

HPS [High Pressure Sodium] is the orange/red spectrum, used in the flowering stage of the plants life cycle.

They each have to have separate ballasts unless you get a HPS or MH conversion bulb [Which ever applies]

CFL's are used in both vegative and flowering stages.
LIP Reviewed by LIP on . Which light first? I'm looking at buying a light which comes with a red and blue bulb. Which bulb works best when? I mean do I use the red first or the blue? And why and which will work best at what stage of growth. Rating: 5