Lint is the accumulation of fibers that slough off clothing, linens and other textiles. When these fibers collect in clothes dryers, the majority of the time the resulting wad of fluff has a bluish-gray tint, despite the fact that your clothes come in myriad shades of green, red, tan, yellow, blue and so on.
The reason for the uniformity of color is that when you mix colors at random you end up with a muddy, dark mess. Melt a box of crayons together and you’ll see this for yourself. But when you mix clothes that are mostly blues, grays and blacks — as is typical with most wardrobes — with a smattering of reds, browns and greens, the mess becomes bluish gray every time.