This tab contains color settings. When you click on any of the fields, a color picker appears: Please note that when you move the most right vertical slider of the color picker - it changes the color transparency and color value in the text field changes to rgba(r,g,b,o) format, where the last number is the color opacity (1 - opaque, 0 - transparent). If you want to make a fully transparent color, move the slider all the way down, or just enter the word "transparent" in the color text field.
These colors options have 2 modes: "Static color" and "Brightness". If you select the "Brightness" mode, you can control high much you want to increase (or descrease) the brightness of color of a region.
If you use an image map, you may want to set the "Regions" color to "transparent", and then set "Select / Hover" colors to "Static color" mode with a semi-trasparent color - so when the mouse pointer is hovered over a region - it becomes semi-trasparent, still allowing an image to be seen from underneath.