If you've not yet heard, the sass functions provided by the compass-colors plugin are moving into the Sass core. Based on some feedback I've received and discussions with Nathan, there are going to be some changes to the existing API.
In particular, the saturate, desaturate, lighten, and darken functions current work by allowing you pass a percentage to change the color by. However, this means that the color you get as a result is not easy to predict and the change is hard to invert. So the plan is to make these functions accept a number between 0-100 but make the change be an absolute adjustment. New functions by the same name but preceded by "percent-" will be added to preserve the old behavior if you're currently relying on it.
>> darken(#777, 50)
>> percent-darken(#777, 50)
>> saturate(#393, 50)
>> percent-saturate(#393, 50)
>> desaturate(saturate(#393, 50), 50)
>> percent-desaturate(percent-saturate(#393, 50), 50)
Please let me know if you have any opinions on this change.