Menu item screen capture results in monochrome image
The screenshot isn't monochrome, but rather the highlight of the menu isn't ‘composited’ correctly in the screenshot. If you screenshot a menu with other colour in it (for example, System Preferences View menu), you'll notice those colours remain and just the highlight changes from blue to grey.

This bug has been around in macOS for as long as I can remember, and has been fixed in the developer preview of macOS 10.14 Mojave to be released later this year.