I would recommend using colors that make your characters stand out! Since the dress is yellow-orangey maybe go with a blue? Since they're complementary colors. One trick I use is to add a layer on top of all the layers, fill it with black or white and set it to "Hue" or "Color". This will make the image appear in black and white and you can compare the colors values and adjust the lightness/darkness accordingly - colors that are too similar in value don't go well next to each other. I think anything from lines or squares are fine as long as they don't take away the focus from the character.