harder water, more blue light in the spectrum.
harder water (in condition that plants can survive first) makes the plants grow slower, thus they have more time exposing under the light per leave. blue spectrum can stimulate more generation of anthocyanin.
high light is not a must. however, if the water is soft, and plants grow too fast (crop per week), then, the leaves have no time to generate anthocyanin. so many use high light to stimulate the leaves for generation of anthocyanin.
If green leaves are changing the color from yellow, then to pink in the middle height of the tank, that may mean the spectrum, and water conditions are suitable. If they are changing color just up from the root, that may mean the spectrum and water conditions are perfect.
personal view. don't take it too seriously.