Near as I can tell, 3rd and 4th edition codices don't mention show it. Those are B&W except for the mini showcases, and Day'leth isn't showcased. I don't have 5th edition codex.
in the Dawn of War video game, Dal'yth's paint scheme is a dark green with a rich purple.
6th edition, the rings around the sept symbols on pages 11, 13, and 18 represent the sept color. Day'leth's is very dark, maybe a grey, purple, or black.
In 7th, on page 17 of the codex, Dal'yth appears to be a dull blue-green. The uniform guide on page 57 shows the same ring around the sept as 6th edition, and the sept marking on the image itself is a blue-grey that is almost the same color as Tau skin.
The color is a little easier to pick out around the sept symbol of an image hosted by ATT's own ARCThunder. http://i290.photobucket.com/albums/ll26 ... 1b3df1.png
Dal'yth as purple appears to be from the video game, and may be artistic license. Contemporary and more official sources show it as an ash-blue. I'm tempted to go through several older White Dwarf magazines to find more information, but time does not permit.
I don't think the DoW scheme is correct, but perhaps we could head-canon it that one sept color was used before Dal'yth was attacked by the Imperium, and another was used afterwards.