My recollection is that phosphorous sounds right for purple. And perhaps nitrogen for yellow. Determinate tomatoes get to be big plants. Indeterminate tomatoes make determinate tomatoes look small (7 to 10 feet). The "pot in a pot" looks small for a big plant. I'm thinking you need to fertilize.

Add some NPK, add some mirco-nutrients if possible, fish/kelp fertilizer is good. Micro-organisms in the soil is good.

Compose is good. Tomatoes can be buried deep, the underground stem grows roots. So at a minimum you can toss a bunch on top next time before you water. If you are going to transplant, mix it in the hole.

I'm trying a dose of old horse dodo this year for each plant. Plants are big, have flowers. It may nave been too hot to set fruit in the past week.

Clay is correct about the purple being phosphorous deficiency.

The other glaring problem is if you had many plants in a pot that size there would be too much competition. Realistically, 10 gal pot is the minimum size for 1 indeterminate plant (I would recommend no smaller than 25 gal. but many people claim success at 10 gal.) Fertilize and cull the weaker plants leaving the healthiest one.

They're starving.

Further, way too many in one pot.

Sacrifice the least vigorous by snipping them off so that you can retain the root system of the one plant in good condition.

Thanks to all of you for your suggestions. I am following your suggestions - applying fertiliser, repotting and getting rid of the weak plants. I will let you know if they survive... Hope they would. Thanks to all of you for your time!

Plant 1

Stiull healthy fruits of plant 1.

Finally, plant 2. Sorry for so many posts.