Different coloured pearls do different things:

Purple/pink: Levels up your octopus~ to get the most out of this give it to the highest level octopus you have, if you wait until you have two of them you can move it up to the next tank and keep pearing it up until the timer runs out or you tun out of octopii (if you have enough coins you can just buy ones you don't have currently in the tank).

Yellow: Gives you a burst of coins every time your octopus evolves ~ this is one of the only pearls you don't want to give to your most evolved octopus, instead give it to a baby and rapidly pear it up as much as you can.

Red: Makes your octopus start rappidly pooping ~ give this to your highest octopus and keep leveling it up.

Blue: Makes your octopus produce diamonds ~ you can give this to any octopus without a difference and it will give you one diamond unless you get the pearl duration bonus up to max then you can get up to three diamonds.