Alright, when you open graphics gale, there is a small box called layer.
In that box there is a down arrow, click it, then go down to properties.
Once you've done that enable transparency and pick a color, after you have found the color you want, click 'define custom colors >>'
Then you want to copy the numbers that are listed by , Red: Green: and Blue:
then click ok.
Lets just say blue is 255, red is 255, and green is 0
Go over to your color palette (make sure you're under RGB, and not HSL)
then take the numbers you copied (which im using blue as 255, red as 255 and green as 0) an it should give you the same color you chose as the transparency...
Then just take the fill tool and fill the background that color and you're done!