Elmo's 7.8 gall tank is still cycling and at the moment he is in Clark Gable's 4g which has black gravel, ludwigia red, water wisteria and a ceramic pot with java moss inside. Clark Gable stands out well in this tank as the colours compliment his but I struggle to find Elmo in this tank. What colours could I use in his new home so that he stands out well, Elmo is Red :) and the substrate is sand.
I took a color theory class in school. I should know this.
What you're looking for are complementary colors - two colors that are directly across from each other on the color wheel. When paired, they create high contrast with each other allowing them to stand out, or 'pop'. Since Elmo is red, naturally I'd suggest green. But not one of equal, or greater, saturation. You'll want to choose a darker shade of green to eliminate any competing vibrancy and distraction from the focal point (your fish). In that sense, though, you could choose any darker color and your fish would stand out, but not in the same way as Elmo would against that 'live-plant' green look if you know what I mean.