Converting 32 bit image to 8 bit palette raster image using FME

In FME you would use a RasterPaletteGenerator transformer, like so:

What I will say is that I don't think GeoTIFF supports a 32-bit palette. i.e. even though I add an alpha band with the RasterBandAdder, and even though the RasterPaletteGenerator creates me a palette with RGBA values, the GeoTIFF output palette only has RGB values.

The log file tells me:

2020-02-20 10:04:22| 8.6| 0.0|WARN |GEOTIFF writer: Unsupported palette value interpretation 'RGBA32'. Converting to interpretation 'RGB24'. If this conversion is undesired, consider changing the interpretation explicitly

So, you can get an 8-bit image with a palette easily enough, but even if your input is 32-bit RGBA, the palette will be 24-bit RGB.