Tomatoes turning yellow from the top down could be experiencing cold nights (do the leaves look a bit blue or purple?) or perhaps the opposite, sunburn leaves (do the leaves have grey or brown marks on them?) either way they should adapt to the temperature extremes soon. A lack of nutrients in the soil and poor drainage will also cause yellow foliage. If you have not fertilized or used a potting soil with plant food than add some Osmocote slow release plant food around the base of your tomatoes now. Make sure the soil is not damp and consider shading the plants with an umbrella for a few weeks if you live in a very hot climate. Keep growing, Marianne Binetti