You want it to be both sprectrums on , there should be a switch on the unit to make both the red and blue diodes light up this is the setting you want it on for flower . Once your plants are done stretching I advise that you turn the blue off and only use the red diodes this will promote denser flower development . The blue spectrum is more less for growth and the veg cycle . I hope this information helps you with your new light !! -Happy Growing!