Details

Prefer color literals (keywords or hexadecimal codes) to be used only in variable declarations. They should be referred to via variables everywhere else.

Bad: literal color

p {
  color: green;
}

Good: refer to color by variable name

$body-color: #0f0;

...

p {
  color: $body-color;
}

Defining colors directly in properties is usually a smell. When you color your body text in a number of places, if you ever want to change the color of the text you'll have to update the explicitly defined color in a number of places, and finding all those places can be difficult if you use the same color for other elements (i.e. a simple find/replace may not always work).

A better approach is to use global variables like $color-text-body and refer to this variable everywhere you want to use it. This makes it easy to update the color, as you only need change it in one place. It is also more intention-revealing, as seeing the name $color-text-body is more descriptive than #333 or black. Using color keywords can obfuscate this, as they look like variables.