Your sentence is very close to being correct! Let's look at it step by step:
Original sentence: "Jellyfish can have different shapes, sizes and colours. But they are all beautiful."
It's good, but to make it smoother and more correct, you can combine the sentences and add a comma before 'and' in the list.
Corrected sentence: "Jellyfish can have different shapes, sizes, and colours, but they are all beautiful."
This way, you use one complete sentence with the word 'but' connecting two ideas properly. Also, the comma before 'and' (called the Oxford comma) makes the list of 'shapes, sizes, and colours' clearer.
Great job noticing how to describe jellyfish!