I could be wrong, but my understanding was that during the first 24-48 hours after hatching, the nauplii were still consuming their yolk sacs and are of the highest nutritional value. Beyond that time when the yolk sac is gone and they start feeding, they are only as healthy as what you feed them.
I'm not sure of exact nutritional values, but I'm sure phytoplankton is one of the best foods for beyond the 48 hour mark. I know yeast works, but they will take any food small enough. Spiriluna powder would be fantastic as well, but slight more expensive.
The key to "enriching" them is to control their feeding cycle with a light (they only feed in the day) or a cover. Light quality isn't imperative to the shrimp so adding light or blocking makes no difference. A few hours before you want to feed them to your fish, you would add light & feed the shrimp whatever food you choose. As far as I know, enriching should be done within 4-6 hours of feeding otherwise the shrimp will digest all the good stuff you have them.
What you use to feed the shrimp to keep them alive is less important than what you feed them to enrich them.