In version 9 it didn’t display color for emoji’s in the subject line of emails when using dark mode (I have dark mode enabled on the OS and in the client/theme). In version 10 there is better support for color however, I’m seeing that some standard emojis (entered via windows key + . menu) still do not have color and are just showing white outlines when in dark mode.
This issue is present on Windows 10 using the latest 10.0.3530 (f0b89b7) eM Client.
Not a horrible deal breaker but perhaps an oversight to be corrected. My image does not represent all icons with this issue, only the few I encountered with a quick check.