Some pics would be helpful. If you can't post pics could you describe in a little more detail the symtoms your plant has. Where is the yellowing? Is it the bottom leaves or leaves all over the plant? The purpling, is that the stems or the leaves?

If it's yellowing bottom leaves and purple stems then the plants aren't getting enough N, now that could be that there is a nute lockout, flush with plenty of water and feed with 1/4 strength nutes.

Good luck!