create transparent colored images

Hello bidders, this task maybe easier said than done. I am trying to make images of colors such as 1 red 1 blue 1 yellow, the colored images need to be transparent or have opacity. IMPORTANT ========= 1 major problem, if the images are overlapped i would like the overlapped color to be the same which ever color is on top. Example ======== blue on the bottom and yellow on the top will result in a overlapped color of green yellow on the bottom and blue on the top will result in a overlapped color of green the same color green as above. I have tried to do this but it seems to result in a different color green see the image for my problem. Images i need ============ I would like 3 circles, 3 squares and 3 triangles 1 red, 1 blue and 1 yellow in each set. image size 150px X 150px and i presume either png or gif format so any uncolored parts will be transparent. Thanks