Contributed by Jos Hirth, with some changes by me:
* Color effects: a new group of extensions in the Color submenu of the Effects menu allows you to adjust all colors of a selection at once. These commands affect both fill and stroke colors, including gradients (but not bitmaps). The commands include Brighter and Darker (adjust brightness by up or down by 10%), Desaturate, Grayscale, Negative, commands for removing or swapping the Red, Green, Blue channels, as well as a Custom command where you can set your own formulas for modifying the color channels. These extensions are a temporary solution; in a future version, similar functionality will be added to Inkscape core.
Adjusting gradient colors may be buggy because a gradient is sometimes shared by more than one object. Also, undoing color changes on gradients exposes a bug where an object seems to "disappear"; this is however only a display issue (probably caused by the order in which gradients and their users are restored on undo) not causing any loss of information. Finally, on large documents and large selections with gradients, Python's XPath code may get quite slow. Despite these shortcomings, I decided to commit this, because it's genuinely useful functionality which was so far missing in Inkscape.