Colors are represented in RGBA, or red, blue, green and alpha (Alpha is transparency). To get their numbers, go into paint or something, then go to custom colors, find the color you want and divide the RGB columns each by 255. For white its easy, on colors[0] change the numbers in the ""s to: "1 1 1 0.4" and for colors[1] change to "1 1 1 0".
EDIT:
Pretty sure it's "0 0 0 (transparency amount)" or something. Not sure though.
Its "1 1 1 (trans)" white is a mixture of all the colors, black is the absence of color.