the retails are fine to install from
the grey ones have to be for you specific model and i mean specific as even all imac g4 look the same ,the internals are slighly different so the grey install disc have to be matching to your g4 imac
to make it clear you cant use a grey disc that came with a 1.25 imac g4 on a 700mhz imac g4 ..it also does not work the other way round with these grey ones as they dont come with all drivers for all processors /graficcards , basically they are lite versions of the osx
so best bet if you are not absolute sure if the grey ones for sale are for your mac
get the retail version ok they cost a bit more ,but they work on every mac apart from snow leopard retail which only work on intel macs