I usually use three color variables ($background, $foreground, $highlight) every time I start a website and expand sparingly from there. If I wanted my headers to have an indented appearance, I can use a text shadow 1px above the text with `darken($background, 10%)` to choose an appropriate color for the imprint without having to manually keep them in sync. Similarly, if I'm ever working with gradients, say on an alert bar that spans the top of the screen, I can simply use `linear-gradient(top, lighten($highlight, 5%), darken($highlight, 5%))`.
If you'd like an example of a website built this way, we completed enyojs.com in about two weeks thanks entirely to SASS and in part to color variables and functions.
As ever, your mileage may vary and what works wonders for me may not fit your style of design.
Of course, once I've settled on final colors, I may go in and tweak all of the shades with exact values.