Semi transparent fill color

[GSAL]

Hi All,
Just started with the Core since a couple of days, but is was wondering if there is a way to make fill colors semi transparent.
I want to use a grid of 3x3 squares with numbers in it and every time a signal comes in one of the queares needs to turn green. But, the number needs to remain visible.
I made it work by first drawing the color filled square and overly that with the number everytime, but the nicer option is to make the grid once and overlay with a semi transparent green square.

Looking forward to your help.