NickS wrote
>  I have a very simple georeferenced tif image file - it has about 8
> different colour categories, which represent numeric ranges. How can I
> convert the colours (which are in RGB format) into a raster, or better
> still into a polygonised vector file? 

Raster -> Conversion -> RGB to PCT will convert an RGB image to a one-band
indexed (paletted) image. You should be able to polygonise this by going
Raster -> Conversion -> Polygonize.

I haven't checked but it might even be possible to polygonise the image
directly, without converting it to PCT first.

N. 




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