The color behavior of the custom entity icons on forms & sitemaps should be similar to what we have for the out of the box System entities for a more enhanced application user experience.

Behavior of the system entity icon ::

Though the color on the sitemap navigation bar is black, the lookup field icon for these system entities would be blue.

The color of the out of the box system entity gets reflected as it is within the sitemap navigation bar as well as the lookup fields associated with these entities.

Expectation :: Just like the out of the box system entities, the sitemap should have reflected black color for custom entities and blue within their associated look-ups used on the form.