cherry tomato with purple leaf
Based on the picture, it does appear that there is a phosphorous deficiency in your tomatoes which can be a result of cold soil temperatures (occurs usually in the spring), improper soil pH (acidity below 6.5 or alkalinity greater than 7.5), extremely wet soil or a deficiency in the soil. The cool soil , pH and wet soil issues will inhibit phosphorous uptake from the soil.
To know more about your soil health we would recommend you test your soil to determine if the problem is a pH or nutrient issue .
Soil test kits can be ordered through MSU at the following link. http://www.msusoiltest.com/
The cost is $25 and everything you need will be provided in the kit. You will receive your results via email within a week or two and will easily be able to see if the phosphorous is low or if the pH needs to be adjusted to get to 6.5 - 7 (the ideal pH for tomatoes). If the pH needs adjusting or the phosphorous is low, there will be recommendations on actions you can take to amend your soil to address these issues.
If the soil test results look good, this may be an issue of wet soil. Based upon where you live, you have been receiving a good deal of rain this year, correct? If so, this means that it may not be a recurring problem.
I hope that helps!