Why does Xcode 11 attribute selector for colors have 2 different colors in the thumbnail?

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.