@Ema_Mavi I don't remember if it was specifically explained this way at any point, but I believe something was said in an earlier chapter that the longer non-magic users are exposed to witches the more they can see of this stuff. So, since Yayoi has been friends with two witches since childhood and knows they are witches, she can see things usually only witches can. I vaguely recall that this was brought up about Kei early on.
@Donutlie In the previous chapter her mother commented on how she's tanned again and warned her to take care of her skin, so I think it is just supposed to be a tan. I guess she's just always been a primarily outdoor kid.