If you live in the US, I strongly urge you to contact your local county extension office for their assistance. This is right up their alley.

You've given us no useful information to go by.....like specifics about the potting medium, fertilizer analysis, and container information.

Purpling of young seedlings is often simply a result of their youth and under-developed rooting system. That in conjunction with overly cool growing temps often results in difficulties with phosphorous uptake. As the plants mature they usually out grow the issue. However if your nutrient supplement can be temporarily boosted in phos a bit it may help.

Dave