From the campaign it was offered to allow the agents to have their facedown cards standing up so they didn't have to pick each one of them up at a time. (also then would be applicable for the kid as well once trust 2+)
Was favoured by some and other were concerned about damaging the cards.
It's optional use.
I believe the colours were to represent if they were admitted or repelled.
Dawei is correct about the use—we recommend using them for the 3 player game so that players can see all of their face down cards at once (don't use them in the 4-6 player game, though, because Agents shouldn't see each others' face down cards)!
We recommend putting the cards in stands ON the board to indicate Admitted, and OFF the board to indicate Repelled. While you can use the colors interchangeably, we use the blue stands for the Kid and the black stands for the Agent.
Apologies—this information should have been in the FAQ, and I've added it now: https://resonym.com/game/visitor/ (click on the FAQ tab)