Purple stems is very common. Its a strain related trait. Its definatly not a problem. Just keep doin what your doin. Check your pH often. Keep it nuetral, but let it rise a little sometimes.

In my outdoor crop last summer here in Michigan, all my plants had purple stems. The purple stems only extended from the leaves to the branches. Some of them turned green, some stayed purple. Never affected the plant at all. As for why a cannabis plant would do something like that, i have no idea. But its beautiful thats fo sho.

One thing tho. I think i see tinfoil in that picture. If your using tin foil and have over 200watts pumping to your plant your not only gonna burn your leaves but yur also losing a lot of light. Tin foil deflects a lot of light because of all the "foils", or the creases, in many different directions. These creases also concentrate light to a point where it will slowly burn the leaves of a plant. I suggest gettin rid of the tin foil. Use some Mylar or even cardboard cut outs around the bottom of the plant painted flat white.

A buddy of mine who grows indoors all year round does that. He cuts out a big piece of card board. Then cuts out holes roughly the same size as all the pots. Then, he paints it white. Once the plants grow up out of the soil and past the cardboard, the bottom of the leaves will recieve a nice healthy amount of light that gets reflected from the white paint.