When using more than one color eyeshadow, always blend the colors together to create a seamless transition from each color. To blend, use a blending brush and use back and forth and circular motions at the line of demarcation until it disappears or is less noticeable. With practice and patience, over time, it will get easier. Many different looks can be achieved when you master blending!
My makeup from today was created with a black and an iridescent white from BH Cosmetics "Take Me to Brazil" Palette and is an example of blending. The blending brush used is also in the photo below and is an ELF product.