When the solid checkbox is checked, the polygon will have a solid color, which you can change by selecting a color from the color box or by pressing the more colors button below the color box.
If the solid box isn't checked the polygon will be transparent, can be altered and removed from a polygon by either selecting or altering the red, green and blue color values.
The lock option will change the three colors together.
Pressing the clear button will create a polygon with 100% transparency, or in other words only the border will be visible, unselecting the border will result in an invisible polygon.
By selecting the border checkbox a border will be drawn, thickness and color can changed with the available options.
Pressing the OK button will store the new color and bring you back to the main window.
The Cancel button will reset the color to its previous value and bring you back to the main window.
The Default button will load the default values for a polygon.
The Reset button will reset the color etc... of a polygon to its previous value.