My main stays in her raiding guild ( and ashamably never wears her guild tabard.. ), however two of my toons on the same server as my main are the sole members and GM of their own guilds. ( for Banking purposes) This means that as GM you get to pick the tabard design.
Wow Vault Tabard Viewer has a little tool where you can play with the colours and multitudes of tabard designs to see what works before going to the Tabard vendor.
Its a pity that we can’t customise the tabards to , not necessarily perfectly accent the fine lines of our latest peice of gear, but more so that at least they don’t cause visits from the fashion police when you wear it. A Guild crest like a military insignia that can appear or not on the backs of cloaks or the front of robes could still identify your guild, but not necessarily cause your eyes to bleed.
My 70 Scrub Fire Mage that I have no idea what to do with yet got her Aldor rep Robe and the tabard of her Guild, Blood Dragons was a red and white Tabard with a dragon Icon. It clashed horribly with the teal and purple hues of the Robe.
So I changed it.
Next up was my Twink Priest Wickedminx @ 211 +healing and level 19 she now has an image to maintain and a reputation on the battlefield to uphold, So like all heros she needed a stylist. She had to have her tabard matching too. As you can see I was going for the green theme. Even her engineering Goggles are green, and the tips of her healing gloves.