Pictures are worth a thousand words.

Contrary to myth, purple stems are not a sign of death or crisis. They can be caused my many factors.

I would be curious to know how your soil feeds automatically. This is not a good thing for our types of tomato plants.

The best way to evaluate the health of the plant from a nute standpoint, is to look at the stem base itself.

Ideally, the stem should be white colored after about 30 days. A healthy white or white wood colored stem base is what you are after.

As for the yellowing tips...could be a zillion things...pics are really needed.

And, dont use 'auto-feeding' soil for these plants. Its great for many plants but some strains of our tomatos just dont like it.

Sounds like a transplant into some Scotts soil or ProMix is in order...use some Superthrive and read my sticky somewhere around here on stress-less transplanting....go another 30 days and then flower.