[gdal-dev] getcolortable is null

[weixj2003ld]

I read a geotiff file, and find that 
poBand->GetColorTable()==NULL
But I use ERDAS IMAGINE9.2 (a software that deal with geotiff), I find that it is not grey,but I use "gdal_translate -b 6 -of JPEG old.tif net.jpg", find that is is grey.
why?
Thk u.
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