The simplest thing to do is just load up a Firaxis icon and see which colours there are bright green and magenta, and then make sure your image does the same thing. In NeoPaint, which I use, every time you alter the palette you have an option of either keeping the same colour indexes (indices?) or changing to the closest one in the new palette, which means it's easy to alter the last two colours without messing up the image.
If you really can't work out the palette, just load a Firaxis icon that seems to have roughly the same colours, copy and paste your image into that icon, and then save it under the new name.