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As requested, here's an attempt to make a new Fortifry back sprite. With the exception of the grayish blues and the darkest brown, the colors on the palette were taken from official Gen. 4 sprites (blues from Lapras and browns from Relicanth) so hopefully they look saturated enough while still staying true to the design.
Included are a few comparisons to some of the fish Pokémon's back sprites, in case others found that helpful.