This was an excellent piece, no doubt, but I feel empty inside.
I feel as if the protagonist learned nothing. That his plot we were told was meaningless. Unless I'm missing the point here entirely, as it is almost 3 AM and I haven't slept in some time.
Obviously you apparently want us to relate with the monotonous drudgery that life is, that I get. Also, the bewildering scope that is our vast nothingness compared to the big picture. Those thoughts were clear. So obviously the nihilistic, or perhaps also even existentialistic, views of the protagonist were clear and I identified well. However, I'm confused as to what we're supposed to take from this as a reader.
Now, I'm speaking from three things here: a loose understand of Campbell (loose in the fact I've only read two of his books once a piece), a personal preference to the way I like to write stories, and lastly my ass (which I do a lot of talking through). So then, correct me if I'm wrong, but he doesn't change. He starts the story wanting to paint the diner red and ends it the same. Now, this could be your intention, and if so, congrats, you made me feel that sense of longing for something more only to never achieve it, which is what the character realizes.
I suppose, to really sum up what I'm trying to say without sounding like I'm being overly critical, which mind you I'm not, I really enjoyed this. What I guess I feel is that, this short story should be more a chapter of a longer story. I feel as if though more could be expanded upon.
Anyway, to leave my post on a higher note: great use of imagery, wonderful syntax, and I loved the piece to a 99%, that other 1% being what I described above. Great read.