If you have a guild that is a different faction than you, their guild tabard will be blank white in cyrodil. For example, I have a guild that was made by an EP player. The guild is EP. I am ad, and when I wear that guild's tabard in cyrodil it is blank white. This should have been fixed back when update 3 came out. This issue has been around since then.
Fire Cloak - VR12 DK - Nord - EP
Ámeer - VR15 Templar - Imperial - AD
The Mágician - VR16 Templar - Imperial DC
Magíc - VR16 DK - Dark Elf - DC
Àmeer - VR16 Templar - High Elf - DC
ámeer - VR16 Templar - High Elf - AD
Æ ámeer - VR16 Templar - High Elf - EP
Ameer Flow - Level 34 Nightblade - High Elf - EP
Youtube:
https://youtube.com/channel/UCFNmXCgmTVo-T-p1BIVLxbQ