The grey allows brighter colors to emphasize the message. This bright blue makes the rest of the words more difficult to read. Do not overwhelm an audience with bright, strong colors all over the graphic. They will pay attention to the colors, not to your content. This provides contrast, allowing colors to emphasize or bring attention to a particular thing.
Nature often shows light and dark colors working well together.
The dark background causes confusion by not allowing adequate contrast with the other dark objects. Bright or dark colors mixed next to each other tend to confuse viewers, sometimes to the point of causing eye-strain. Background Background, large-area or base colors should be quiet, soft, dull, muted, greyish, neutral colors that allow smaller, bright areas to stand out vividly. The background in this image allows the viewer to concentrate entirely on the foreground. Although the foreground is still clear, the bright background distracts the viewer.