A few moments later in the same scene, a red trash can comes into view (lower right of photo). Also note the red 'Aaron Brothers' sign across the street at upper left. Only the portion of the Yellowstone sign that says 'Yellow' or 'Yellows' is now visible (just above center in screencap - click to enlarge).
The prostitute prepares to climb into the back of the blue van. Recall that earlier in the analysis, we said that the hit man's blue key opens the back door of this van (or at least, it did until the van was broken into or hit in an accident, as evidenced by the damage on the door; now the key is no longer needed, since the lock has been broken). The overall idea here is that we're supposed to associate redness with prostitution, and the blue van with Diane's alchemical rubedo, as described below.
Recall from our discussion of Diane's alchemical stages in part 11, that we said the fourth and final stage, the rubedo, represents the Self archetype. As we said, the Self manifests itself in "wholeness," a point in which a person discovers his or her true nature. What's being done in the scene shown in the above screencaps, is that the final stage of the alchemical process, the rubedo ('the reddening'), is being associated with the final stage of Diane's individuation, her Self, via the blue symbolism - the blue box represents the blue van, and the magical blue key represents the van door key. Near the end of Diane's dream-sequence, which comprises almost the entire first two hours of the movie, the blue box is opened and the Self is revealed: Diane's true nature is that she is a prostitute. It is at the point of this realization that the box hits the floor, and Diane awakens to reality.
