im assuming we are chatting about LED lighting. im only a newbie so i might not be totally correct but blue is generally for veg and red is for flower. i dont think the red has much effect if any on the veg stage. but ive read that the plant can absorb all different spectrums of light such as IR, UV or green/yellow spectrums which enhance or effect most aspects of the plants quality,growth or flowering. although im not sure by how much. my light has a combined VEG&BLOOM switch with blue, red, green, IR and possibly yellow which will cover all stages of the plants life :) i hope i helped man and if you have blue and red on and it doesnt burn the tops of your plants then it couldnt hurt to run both at once? Peace!