This worm gives your choco a small amount of health and makes it completly full. On Jeffdogg's post i dont think the parasite worm has much to do with its color, i didnt use any worm and my choco turned out black.
Well for the most part they say it gives the young chocobo more Energy (Health) Prices not to bad on Garuda they go for 5-10k. Honestly I wouldn't waste the gil on that I use Cupid worms and Parasite worms. Cupid speaks for itself it gives the young chocobo a boost of affection, while they say they are not sure about the Parasite worm I believe it may have something to do with the choc's color. This is the first time I used Parasite worms and my choc turned Red :D, my other 3 choc's were all yellow and i didn't use any worms. Just a thought lol anyway hope my info was a little helpful :).
I dont know the efects but I grew 4 of these. I used brass pots and planted wild grass seeds. Planted them on any day. Never fed them, and when it was time to harvest I got 4! Mybe I was just lucky. For the cupid worm I planted in the same pots Wild grass seeds on light's day and fed them light cryst and harvest on darks day. I got 3 to 4 cupid worms. Hope this helps. I think it was down to luck :))