Tomato leaf crud
Purplish leaves in young tomatoes can be an indicator of phosphorus deficiency. Your plant might not be getting enough nutrition if it is in a pot that is too small. Is this a dwarf or patio type tomato? If it is a regular (full size) tomato, we recommend a 5 gallon pot for each tomato plant. If you have several plants crowded in a pot, there will be competition for root space, water, and nutrients. You could transplant to a different pot. Some potting soil mixes already contain a general fertilizer. If yours does not, you can add a general 10-10-10 fertilizer and see if the plant improves. Also, make sure the plant gets full sunlight and sufficient water during very hot, dry weather.
Here is a resource on some of the common problems that occur in tomatoes -- and the causes.
For additional tips on growing tomatoes, see the following web page: