There are a few overlapping issues present, one of which will be a quicker fix than the others. The missing leaves are due to a caterpillar called the Tobacco Hornworm, a very common insect pest of tomato plants. (Despite their names, between Tomato and Tobacco Hornworms, which look very similar to each other, the Tobacco species is more common on tomato plants.) We've attached a copy of your photo with the hornworm itself circled and another of its telltale signs (other than missing leaves and chewed-into fruits), which is frass (insect poop).

All you need to do is pluck the caterpillar off the plant and leave it somewhere for birds or other wild animals to make into a snack. If you don't want to kill it, it might be happy eating petunia plants since they are a member of that plant family, but left in place, the caterpillar can do significant damage to a young tomato plant because they eat so much. Their large size and strong sucker-like feet might intimidate people, but they won't hurt anyone, though sometimes they poop on you when handled for too long. If you don't want to pick it up and it's on a leaf or side branch end, you can snip the leaf/stem off instead with the caterpillar still holding onto it to move it elsewhere.

The leaf yellowing and purplish tones may be due to nutrient deficiency stress. Is the pot being fertilized regularly? (How often depends on what kind of fertilizer you use.) Daily watering can be good in this high heat, but once the heat wave abates a bit, make sure the soil is monitored before watering so you only water when it's needed. To be fair, most tomatoes in pots will need watering daily by midsummer, but a lot can depend on how well-rooted the plant is, what size the pot is, and the soil type used. You can let the soil get somewhat dry to the touch between waterings about one or two inches deep, but once it reaches that point, it should be watered well enough that the excess freely drips out of the bottom drain holes. Over-watering can lead to similar symptoms of under-watering, like wilting, and it can also remove nutrients from the potting soil faster.

If you need to fertilize, follow package instructions for dosage, and don't worry about finding a precise formulation marketed just for tomatoes; anything that contains most or all of the typical plant nutrients should be fine. These pages might be useful:

• Growing Tomatoes in a Home Garden (it focuses on in-ground growing, as that is the ideal, but can mostly apply to container-grown plants as well)
• Fertilizing Vegetables (again, focused mostly on in-ground growing)
• Maintaining Container-Grown Vegetables (includes fertilization tips)

A few comparatively minor pests like spider mites and flea beetles can cause leaf damage, though we're not certain if they are present here since we can't see clear indications of their presence. You can explore the Key to Common Problems of Tomatoes page for information about these and other issues that often arise, plus how to manage them (or at least prevent them from worsening, if treatment would not be effective).

Miri

You're welcome.