Are they planted in pots or in the greenhouse soil? Is your greenhouse unheated? Have there been frosty nights recently?
Tomato plants affected by cold, rather than actually frosted, go a sort of bluey green colour and look quite unhappy.
Lower leaves going brown and falling off can happen where the plants have dried out or are suffering some kind of deficiency. Overwatering can have a similar effect.
I'd suspect that they are just unhappy being planted out now while it's still quite early while the nights are still very cold. Either that, or there is something in the compost which is not good for them.
Personally I won't be planting my tomatoes in the greenhouse bed until early May and when they are already in flower and beginning to set the first truss.

I agree with you Bertiefox, it seems rather early to me too.

I agree with Bertie too. I suspect you've bought your toms from a garden centre. They would have been raised in humid heated glasshouses and havent been hardened off before being planted out in your cold greenhouse with it's heavily fluctuating day and night temps. They will probably recover so long as you keep them warm until the night time temps improve. Try covering them with fleece during the night.