tomato leaves turn yellow and purple
I am an amateur gardener. I have some pots in the balcony and I grow flowers and herbs like mint. I do not have problem with them. A month back, I planted some tomato seeds (just the seeds from store-bought tomatoes) in a pot and used the normal soil got from a supermarket. It says that nutrients in it are sufficient for 4-6 weeks. But, the supermarket lady warned me that it will not be sufficient for plants yielding vegetables and fruits and said I need to top up with fertilisers later.
The tomatoes plants grew; there were many plants in the same pot and all was well till a couple of days back. The leaves at the bottom of the plants started turning yellow. Very soon, the whole plant had a purplish hue. And the underneath of the leaves (veins) turned dark purple. A couple of leaves have shrivelled up almost died. In some places, I can see yellow and purple patches. I googled for info, but I am unable to decide whether it is a phosphorous (or some other) deficiency or a disease. Could anyone here kindly help me?
The temperature here varies between 10 and 25 deg C now.
Thanks in advance for all your replies!