My headcanon is that Siuan is very handy with a needle. However, Cairinhean patterns are very hard to get a hold of - you wouldn't want random people knowing where the weak spots in your armour are, so to speak. It takes her a while, but Siuan finally gets her hands on a single pattern such that she can make Moiraine a dress. She sews the white one when they are novices. It's tricky, and she doesn't quite get the hang of it, but Moiraine loves the first draft so much she wears it to sleep every night. When they're raised, Siuan makes her the blue one off that same hard-won pattern. She's had much more practice now and it comes out as fine as you'd find in any dressmaker's shop. Just with quite a bit more love sewn into the seams.
