color-shifts.less

.background-color-shift (@color, @amount: 10%) when (lightness(@color) > 50%) {
	background-color: darken(@color, @amount);
}
.background-color-shift (@color, @amount: 10%) when (lightness(@color) <= 50%) {
	background-color: lighten(@color, @amount);
}
.text-color-shift (@color, @amount: 10%) when (lightness(@color) > 50%) {
	color: darken(@color, @amount);
}
.text-color-shift (@color, @amount: 10%) when (lightness(@color) <= 50%) {
	color: lighten(@color, @amount);
}

I saw in the post that you need to stick with Less for this, but are looking at Sass in the future. If you do end up giving Sass/Compass a proper go, you should check out a project I work on called Compass Recipes. I added a ["color-by-background"]( mixin, which you can call like this:

.myelement {
  @include color-by-background($bg-color, 40);
}

Where 40 is the amount of "contrast." Important to note that color-by-background also uses a custom "brightness" function, which is better at checking background readability than Sass's built-in "lightness," and also has option for $inset-text (adds a small text-shadow, varying light or dark). Here's a demo page that shows what I mean. Hope that helps!

replace <= with =<.

In less the comparisons match on this: /^(?:>=|=<|[<=>])/

Thanks, @fat. I forgot LESS comparisons worked that way.

No worries :)