I am making a cake with two fish that look kinda like this
I've never tried this myself, but I've heard of others using a "toothbrush painting" method where you dip the toothbrush bristles in your food coloring and scrape the bristles against a metal mesh strainer to get the sprayed look. Since the base fondant would be colored, you could just do the green and darker blue spots with the toothbrush spray.
Otherwise, you could try using a sponge.
I do a lot of painting and airbrushing and painting *with* airbrush colors (not with the airbrush) and lots of shading with colored food powder. Might be better off dry dusting with food color powder, and going over that with luster dust for the shine. Use a big blush type brush for the luster, and use smaller soft rounded ones to blend your colors. You can find dry food color powder lots of places on the web.
And get an airbrush for next time.
I use to use these before I got an airbrush.
you can get there with the powders as has been mentioned--
i would start with blue fondant too.
if you want a really bright sheen on the fish scales--you can get edible glitter--the really light weight light as a feather stuff--dissolve some in water and paint it on--test it on something first to make sure it doesn't interfere with your coloration
in the states--you can get this at any michael's or hobby lobby type store--of course at cake deco stores
also known as gum arabic
the powdered gum arabic works the same way
this might make it a bit more shiny than it already is (but fish are pretty shiny i guess)
just a thought