I would suggest to use the symbolic variant of the media eject button, even when keeping the rest of the icons colored (see patch attached).
Created attachment 9315 Patch with symbolic icon name
And just to be sure: I checked that the icon is part of the Adwaita icon theme
Fine for me ! Tested as well Tango, Breeze, Breeze-Dark, Oxygen and GNOME, works fine for all of them. To my surprise "locate media-eject-symbolic" only showed an entry for Adwaita and breeze. So I assume if the symbolic one is not found, the standard "media-eject" will be used as default.
Simon Steinbeiss referenced this bugreport in commit b5daef31fb23f3e28a461afe69d0ca5f032c8212 Use symbolic icon for eject (Bug #16281) https://git.xfce.org/xfce/thunar/commit?id=b5daef31fb23f3e28a461afe69d0ca5f032c8212