Ok perhaps I am getting your issue wrong.
Are you trying to set a MATTE ( which has to do with the handling of semi transparent pixels) or a color table.. which has to do with the of colors the gif contains ( but not how it handles semi-transparency).
in other words, for you to see "red" in the "matte" area" of the actual image at least some of those pixels have to be semi transparent.
IF on the other hand what you are trying to do is posterize the image ( reduce the total number of colors)
You can could:
1.set colors to 4, the color table will be reduce to the 4 closest colors to the actual image.
2. then double click on each color on the color table, it will lead you to the color picker (not the swashes ... as I said before the save for web is NOT swash based. ) At the color picker you can then enter the values that correspond to the swashes of the colors you want.
I hope that helps.