A mix of light spectrum is better than having just one and then just another...having both spectrums throughout is the ideal, but in veg the plant uses more light we see as blue, and in flower they use more that is orange/red...I have successfully flowered some killer buds with just a MH...and now I use a CMH...Metal halides are the blue spectrum that is supposed to be for veg, but they also flower with super awesome results...HPS is orange and for flower, but some people have successfully grown awesome buds using HPS for veg and flower...so to each his own...but plants ideally have all spectrums available...just like the sun...they just use more blue and orange and we capitalize on that, but it isn't written in stone...I have never seen the sun shine orange, or blue intensely for any length of time, and it has been growing plants successfully for millions of years...