You can change the Start menu to any custom color you want, but that will, by default, change the Taskbar to the same color.

You can enable the refreshed Start menu on Windows 11 after installing the KB5067036 update and enabling a feature ID via ...

Unlock Windows 11's new Start menu in 24H2/25H2. Learn how to enable the redesigned layout, features, and fixes for a smoother experience.