There are other Paulson casino chips that exhibit slight variation in colors. Empress casino come to mind. See pic below. All these Empress chips are in the same, cleaned, unoiled (?), condition. There are color differences in the red $5, the green $25, which is the most noticeable, and the green and pink spots on the black $100. I've assumed this to be color variations in different batches of clay when Paulson made the chips, even though they are technically the same 'Paulson color.' (i.e. Hot Pink) For example, if Paulson makes 20,000 $1 chips for a casino, it'll likely be in multiple batches.
Also, some Vegas casinos bring out brand new chips every once in a while (I played with brand new chips at poker rooms at Bally's, Venetian, and Golden Nugget poker rooms a few years ago), but I don't know if these were different batches/orders they've bought from Paulson, or just cycling through all their old stock of new chips.
IMO, discoloration due to fading would most likely be non-uniform on the colored edge spots and edges, except if chips were placed in some sort of liquid (coffee, coke) or cleaner for an extended period of time.
Also, if two of the chips are dirtier than the others, it could be that natural oils that have darkened the color. Some Paulson colors look much more saturated when oiled.