A simple rule: in order for a banner to be noticed, it must be different from the site.

There is a simple rule - if you dress differently than everyone else, then you will stand out from the crowd, and you will be noticed. Suppose you are in the company of girls in pink dresses, and he is dressed in a black suit. Some man enters. It is clear that he will be more interested in girls, but in any case he will notice you too.

The same can be applied to banners. For example, Habr is gray-blue, which means that orange (opposite color) banners should be placed on it. If you squint, then Habr is white, which means you need to fill banners with color. Etc.

Added by:
I am not writing about extremes, when the banner overshadows everything else, but just pay attention to the general rule.