If you've got Photoshop you can try another route, but I've had mixed results on anything with faction colours. I keep meaning to try out the PurpGrab method but haven't quite got around to it yet

Generally for reviewpanel icons, if I'm modifying an existing one, I export to BMP with BIFreader, open in Photoshop and again save the palette (it's in IMAGE > MODE). Then I do the editing as required, reload the colour palette (or convert to indexed using the saved palette if I've had to change it to RGB to do the edits), save as a BMP (again 256 colour) and put it back into BIFreader - but there can be occasional black spots. I find it helps to set the second colour in the colour table to something outrageous that you won't use (I go for turquoise ). Do this by opening up the colour table of your image, double clicking on the second colour (ie colour 1, as it starts at 0), then setting the RGB values. As far as I can tell this seems to be some sort of secondary (or possibly part) transparency colour, hence why I chose a colour that won't appear in the actual image.

If your background came out not transparent it seems like the colour value might be slightly off - check the colour table of your BMP (in Photoshop) and if necessary adjust colour 0 to "008000" in the HEX box (IIRC: 0,128,0 in RGB), resave, and re-BIF-ify it.

It's definitely worth investing some time in studying Photoshop's colour mode menu, and building a library of colour palettes, saves a lot of heart-ache in the long run