When the Buu gives you an option to Guard or Calm down it means it can change color.
After you do a few missions they will return and you can collect your item. In order to get the little Buus moving you have to feed them once, feeding them 5 times changes their color. If you are a Buu you can have 10 but if you are anything else you get 6 Buus.
I am going to make a list with the Buus I create but my guess is the rewards are given at Random. Red is a Super Soul, Blue is a Recovery Item and Yellow is Clothing for a reward.
