According to the new Seed Starters Handbook, purple-leaved tomato seedlings are most likely to be suffering from a phosphorus deficiency. To encorporate trace minerals into the soil for young plants, I like to grind up a Solgar Total Minerals tablet and some egg shells for calcium....since I have been doing this and adding it to the seed starting medium (which is mostly vermiculite) I have not had any problem with mineral deficiencies.