Basically, convert your image to greyscale to check if you've got enough shadows and highlights to make the image pop.
Don't let complementary colors (colors opposite each other on the color wheel) trick your eyes!
Converting to greyscale also helps by taking out the distracting colors and helps you see if you've got the lighting and focus right. If it looks ok in greyscale, it will look ok in color.
Though using complementary colors do help make your character pop, so you can consider using this also.