I gave my fabrics a trial with a few shades of organza. That was a no brainer as it would provide the shimmer I wanted on the fish. Red was best over my orange fabric. Black over the orange worked for the belly fins and the tail.
After making a drawing, a copy and a reverse copy on the fusible web, I cut out the body and then carefully added the organza a tacked it all around to the fusible coating.
For the upper fins I used a pale grey. The head is the only portion without organza but that will be gussied up with thread. And I may change the organza colour on the tail......too dark.
So this guy is about 18 inches. I'm rather please with my fabric efforts.
(PS I just went out and ordered a bolt of white fabric and 4 litres of the primaries. My husband is such an enabler. It was he who pointed me to the product.)