It could be two things causing this yellowing,
One is over watering and letting tom'e sit in a saucer thats full of water and also the pot itself is soaking wet,

And of cause if the tom's are not watered on a regular basis this can cause the plant to die,

But another cause can be lack of a good feed, "tomato feed"
is high in pot ash and pot ash feeds the leaves,

So you've these two area's only you know whats going on,
IE are you over watering or underwatering or not watering on a regular basis?

And are you feeding the plants,

And of cause did you re-pot from seed compost to a higher compost, "remember seed compost has very little fertilizer in its make up.
All my tomato plants are in an unheated greenhouse and ive no yellowing problems;

Just one last thought that may help,
If you find its over watering and these plants are in a plastic pot you'll understand why a lot of gardeners use "clay pots rather than plastic"
The reason being when watering a clay pot your not just watering the plant when you use clay pots,

your also watering the clay, the water will soak into the clay and as the days heat rise the clay warms and this acts as an insulation to the pot and also evaporates the water, Hence no water build up for the plants to drown in.

Plastic just lets the over watered plants saucer stay full until the plant drinks this water "but if the plant's composts is already soaked!!!!!! the roots are also soaked and the leaves turn yellow and fall off.

To check if your giving the pot enough water it's a case of looking at the pots top rim area, this will show you how far inside the pot your compost should be and no higher than this rim, the area from the top of your rim to the bottom of the same rim is the amount of water you should be filling that size or pot with and no more.

One other point that may have happend is again to do with pots and the different kinds of pot you can use,

A lot of clay pots do have only one hole in the bottom and its normal to place bits of broken clay pot pieces over this hole to aid dranage and stop compost both falling out and blocking the hole,

Other pots both plastic and clay may have 5 or even 7 holes in the bottom of the pot, these do not require any bits of anything to aid drainage as with this amount of holes the water will drain through,
so have a check that your not drowning your plant by bad or no drainage in the pot.

A few things to think about here "but only you know whats the possible answer.
Regards dungy.

thanks for that dungy,lots to go on there,i re-poted them from 3 inch to 4 inch pots and they have never looked good for weeks and yet i have another variety in same size pots and they are doing fine, eventually when they are big enough i put them in greenhouse border.