Ubuntu – How to change the bucket fill colour in Gimp Image Editor

It sounds like the image you are editing is using a limited colour palette rather than full 24-bit RGB. GIMP is doing its best to do what you ask while using colours in the existing palette.

If you want to switch the image to full colour, select Image -> Mode -> RGB from the menus. You should then be able edit the image using any colours you want.

If you need to save the image with an indexed colour palette, you can switch back again by picking the Indexed... menu item from the same menu. This will pop up a dialog box asking what sort of palette you want to use for the image. The default option should suffice.

If you don't need an indexed colour palette, I would just leave the image as RGB. For many of the images people use PNG files for, the savings are small.

It's normal. From the GIMP documentation:

2.14. Close

The Close command closes the active image. If the image is not in the main window, then the window is closed also. If the image is in the main window, the image is closed but the window remains, empty.

Emphasis mine.