I have had my standard L. aromatica grow in the purple/red growth form too (but it never did it regularly, just ever so often). I tend to think it depends on the lighting and tank conditions, but maybe others will disagree. I think it is just very variable in growth form. Actually the plant I have was sold to me as L. hippuroides which is the same as aromatica.
I really believe (know) that the color of the Plant depends on the type of bulbs that are being used. I have both Hipporoides and Aromatica in my 150 gallon tank. I have the front of the tank set up with the GE 9632 (?) bulbs and both plants are Red and Purple , colors that glow like you would not believe.
In the rear of the tank I have 6500K bulbs, which are more Greenish Blue tinted than the Pink 96s. In the back of the tank both plants are Glow in the dark, toxic green on top and very pale pink.purple underneath. Both are recieving the same exact amount of watts, just different color spectrums.
I have seen hipporoides that seem to be glow in the dark purple and have fell in love with "that" color. I wish I knew what type of bulbs that person was using. I can not remember where I saw the plant, On auabid or one of the other sites. I am sure you all know the PIC. If you know who was growing that, please let me know.