Heather grays are the classic example: dark and light fibers spun together read as a soft, textured gray. Many heather colors also come from blending cotton with polyester or rayon, then dyeing the fabric so each fiber picks up color differently. That is why heather labels often list a blend, and why fiber content can change from color to color within one style.
For printing, the blend matters more than the look. Polyester in a heather can bleed into light inks during curing, so printers watch for dye migration. Water-based and discharge inks soak in unevenly, which gives a vintage texture. DTG prints look softer and less saturated on heathers than on solid cotton.
Check the content line for the exact color you are ordering, not just the style. A heather gray, for instance, may contain some polyester while the solid colors in the same tee are 100% cotton. See how to choose a blank.