Blue and Ivory

I've noticed the repetition of the phrase "blue and ivory" several times in the book, but I'm still not sure what it means. I've counted the phrase "blue and ivory" five times, and "ivory and blue" once. Most of the time these colors were describing bare feet.

When Billy is writing about the Tralfamadorians as an older man: "He was barefoot, and still in his pajamas and a bathrobe, though it was late afternoon. His bare feet were blue and ivory" (28).

Right before Billy is kidnapped by the aliens: "Billy Pilgrim padded downstairs on his blue and ivory feet. He went into the kitchen, where the moonlight called his attention to a half bottle of champagne on the kitchen table, all that was left from the reception in the tent" (73)

When the door of the boxcar is opened for the first time: "Billy Pilgrim was lying at an angle on the corner-brace, self crucified, holding himself there with a blue and ivory claw hooked over the sill of the ventilator"(80).

At first I just thought that the author was trying to say his feet were cold, but with all this repetition I'm thinking that there's a deeper meaning.  In The Great Gatsby we talked about the color symbolism of the green light, so I thought this might be similar. From what I found, blue meant tranquility, stability, wisdom, heaven, etc. White meant purity, innocence, goodness, etc. I couldn't really find a strong connection here. Another thing I thought was that blue and ivory had to do with death, because that sounds like the ashen color of dead flesh. The funny thing is that "blue and ivory" was used to describe both dead and alive people. Maybe it's foreshadowing how close Billy is to death at certain times. 

What do you guys think? What might this be a symbol of? Do feet have significance? Is the order of the words (ivory and blue vs. blue and ivory) significant?