What you can do with RD-D
Here is a simple example of what you can do with Rubber Dub-Dub. The rubber tool is changing the hue of the woman's hat, but the shading and other characteristics are preserved. Note also that although the rubber is used carelessly beyond the border of the hat, the colours in the background stay as they are.
Of course, you may not want to change the basic colour. Perhaps you only want to change its hue slightly. Or you might want to make the colour stronger or brighter for example, or even change it into greyscale.
These simple tasks are not so "simple" as they seem, and you will be hard pressed to find a piece of software that does this conveniently. That is why Rubber Dub-Dub was written for you.