When Tropical Fish spawn in a warm ocean, they will have their colour, pattern and shape randomly assigned. You can collect them in the bucket and the name will show the correct details.

As soon as you empty the bucket out, with the fish, it loses all of its' colour and becomes gray/white. It retains its shape and pattern however.

To recreate:
Catch a spawned (natural or spawn egg) tropical fish in a bucket.
Release it.
Observe loss of colour.