In clothing, heather refers to interwoven yarns of mixed colors producing flecks of an alternate color. It is typically used to mix multiple shades of grey or grey with another color to produce a muted shade (e.g., heather green), but any two colors can be mixed, including bright colors.
Sometime also referred to as a frosting color.
Remember our traditionally old gray sweatshirts had that speckled 2 color look to them?
Well now they have done the same look with colors, we have the red, russet and military green in the heather look.