Definitely dirt
That bumper is supposed to be sort of a dove gray, almost white. My G3's bumper looked like that when I first got it, although my red tape stripe is faded pink. It's hardly perfect now, but it is closer to the original color. It still has some dark spots that almost look like mildew stains. The cord looks like it's nicotine stained, but that's apparently a natural discoloration. Some of the other plastics are yellowed as well. G3's generally don't age well, appearance-wise. It's little wonder that the G4 had charcoal gray livery and every Kirby after that has had a darker color scheme.
I agree that G-series Kirbys get a bad rap for their weight and complexity. People just need to learn to use them properly. They are absolutely cleaning BEASTS. I snagged an almost pristine looking G6 with its toolkit at Goodwill one time after a couple walked away from it. The wife dismissed it as "too heavy" after trying to push it, turned off and with the TechDrive engaged. As soon as they were clear of it, and looking at some plasticrap Bissell or Dirt Devil, I Grabbed it and headed for the checkout before they had a chance to change their mind.