meimichi

I'm thinking that they were probably doing it for the symbolism there; remember Holly was setting up that it was a universe where Chloe was Lois Lane minus, obviously, hair change, and while Clark clearly hadn't married her yet and seemed worried about ever doing so (so clearly, it places Lex as the reason they are OTP which is a great link back to the visuals of Tempest so I really love it) they really obviously directed AM to hide her ring finger, and the back touch, and the dialog setting up Clark helping her find her tree & working together & leaking to Ausi that Chlark were "living in sin"...

I think it was done to paint her fully as Clark's girl from the drop, as he's in solids and she's in plaids. They both wear red (traditional holiday) but he brings in the blue and she brings in the yellow making them united in Supermanliness.

Which, putting her in plaid there makes sense as they had already clearly set it up in Hidden with Lana and her scenes with the Kents and with Lex, and they had teased it a bit in Gone prior with people's assumptions.

So my guess is plaid = real Clark's girl / human Clark's girl. Which is why it was used with Lana and used with Chloe especially in like s9. But it's clearly become ~a theme~ from there on out, much like the use of the primaries and the use of the secondary complimentary color combos. *shrug*