You can take your picture there, open it up in a paint/graphics program. Anything would work, even MS paint. Although the better programs will give you more options to play with though. Anyway, just colour in the tabard with different colours until you find one that almost works. Then adjust the colour around until it\'s perfect, lighter, darker, adjust the mix (I.E. if it\'s a green, try using more of a bluish or yellowish green...or play around with the mix of the highlights and shadows...highlight with greens or yellows...shade with green, blue, black, red...whatever).
There\'s an army painter thing on the GW site too, I know there\'s a Marine one, I think there\'s a Chaos one. You could check it out too, not sure if it has a tabard.