I closed the window for picking colors in Kolourpaint. How can I get it back?

as best as I can tell you need to close kolourpaint then remove the kolourpaintrc (/home/smiley/.kde[4]/share/config/kolourpaintrcfile) or minimally remove the state= line in the file

I would suggest filing a bug report if one doesn't already exist

The developer informed me that it is possible to get back the color box by right-clicking on the menu bar. Then you get a sub menu and tick the first entry. I didn't find it on my own, because I only tried right-clicking on the tool bar.