For a two color theme, you may choose either analogous or contrasting colors. Sapphire blue and turquoise are both in the blue family, but turquoise is shading towards green. Sapphire blue and violet are both in the blue family, but the violet is shading towards red. Red and purple, gold and green, peach and pink, wine and rose, are other attractive analogous combinations. Contrasting colors are not related and, I think, more diffcult to combine attractively.
Examples are blue and orange, red and green, yellow and blue, or red and blue.
Black and white is the most extreme of contrasts, and needs to be worn by a dancer with dramatic coloring and bold presence. A pastel mixed with a contrasting vivid color is visually disturbing, to my eye. If you find attractive color combinations on a patterned fabric, you could choose those colors as a theme for your costume, using the fabric as a skirt or other piece. As always, I'd advise anyone in doubt to keep it simple, love your colors, experiment with swatches, and ask the educated opinion of your teacher or a fashion-conscious friend. Click here to ask the Costume Goddess your own question about belly dance costuming.