Striping is the easiest way to create a good looking fish hat. Just pick the colors you like and knit the stripes up while you go. Or...you can use a system:

1. Roll the Dice method;
2. Fibonacci striping method
3. Random Stripe Generator http://www.kissyourshadow.com/stripe_maker.php
4. use self striping yarn
5. combine self striping yarns, uni yarns and one of the mentioned striping methods.

1. Roll the dice striping
Take a dice and let it decide how many rows you knit in one color. Roll a 4, knit 4 rows. Change colour, roll a 3, knit 3… You can also let the dice decide what the next color will be. Pick your colours and give each a number. First roll the dice for the colour, then for the number of rows you’ll knit.

 2. About Fibonacci striping:
Each stripe has as many rows as the sum of the two preceding stripes. Starting with 0 and 1 the striping goes like this: 0, 1,1,2,3,5,8,13,34 and so on. For your Fishy 34 and more rows may be a bit too much. From here you can count back, or start again with 1, 1, 2 etc. Visit wikipedia to find out more about Fibonacci.