If the screen doesn't exist when AWeb starts, the default public screen is used instead.
Clicking the button will pop up a standard screen mode requester. Here you can select a screen mode, width, height and number of colours.
With this chooser gadget, you have the choice of loading a colour palette, a grayscale palette, or no palette loading.
AWeb will never load such a palette for screens with more than 256 colours. Those screens will be on a graphics card that is assumed to accommodate "enough" colours for all possible images.
Please be aware that 256 colours (or less) is not many. Although AWeb tries to calculate an evenly spread palette, still many images will not look optimal. If you don't load the palette, the first few images look great, but later images may look even worse. A grayscale will give more acceptable results when there are little colours available.
If you select a grayscale palette, you will probably want to configure your JPEG (JFIF) datatype to use a grayscale mapping for the "AWebIP" task. Otherwise JPEG images might not be displayed.