Colorfield class creates a rectangle filled with a specific color, useful e.g. within a palette requester. You can change the color of the field at any time by setting its RGB attributes.
The field will try to obtain an exclusive pen on the current screen. When none is available, it just displays some kind of rastered background. Maybe it will get a little more intelligent and try to display the color by mixing together some other colors, but that's a future topic.
Needless to say that Colorfield only works with Kickstart 3.x and above, since lower operating systems don't support pen sharing. When using this class with a lower OS, you will also get some kind of (boring) raster.
Here is an example XML excerpt for creating a colorfield gadget set to all white: