Here is an image of the color selector in the attribute inspector of Xcode 11.x. I could not figure out why a lot of the color thumbnails show 2 colors. I suspected that the pair of colors indicate the Light and Dark Modes but this does not seem to be the case.
Those colors have an opacity less than 100%.
The left side is what we see when the color is on black background. The right side is on white background.