On my phone, I see a shaded black diamond:
This indeed looks the same as the diamond after moderators' user names does on my device. It also matches the appearance of the other black playing card suits in my comment above. I thus conclude that tha SE Android app isn't actually doing anything special about the ♦ character, but just letting your device render it as usual. It just happens that your Android device is choosing to render it as a fancy colored (or, in my case, shaded) "emoji-style" character, rather than as just a flat black geometric shape. The exact appearance probably depends on your Android version and any customizations your device vendor may have made, including the set of installed fonts.
Anyway, perhaps the real issue here is that the app should be rendering the real mod diamond as some custom icon, instead of just displaying a Unicode U+2666 Black Diamond Suit character and letting the OS render it any way it likes?