A trademark can run out if no longer in use by the one who trademarked it. In DiMarzio's case, doesn't look to be anytime soon.
The same way smells can't be patented, the formula can but the perception of what something smells like can't. That's how knockoffs of cologne and perfume exist. With colors of plastic, by merely changing one of the dyes a small amount, it would now be a different color to a chemical test, but not to your eyes. UPS has trademarked that color Brown and Richard Petty has trademarked his color of Blue - that is the specific formula but not your perception of it. Anyone else can make a near same color by altering the formula and be in the clear. In other words, DiMarzio may have the Cream colored [humbucking] pickup market cornered, but not the Ivory, Off-White, or Vintage White colored pickup market.