The red purple stems equates to a slight sun burn on your plants from your powerful lights. It's not a cause for concern unless leaf or bud bleaching show. New growth is usually a lighter green when your plant is healthy and growing rapidly! Don't worry they looks amazeballs!!!! Great work! Happy growing!
Also they have also got red purple stems when last week they never I only check the ph not the other as my pen is a weird one and I don't under stand it.