RGB stands for red, green, and blue. d.rgb visually combines three raster maps to form a color image. For each map, the corresponding component from the layer's color table is used (e.g. for the red layer, the red component is used, and so on). In general, the input raster maps should use a grey-scale color table.
If color quality of satellite image color composites seems to appear poor, run i.colors.enhance on the selected satellite channels.
An alternative is the assignment of grey color tables to each band with r.colors:
r.info -r image.1 min=0 max=255 r.colors map=image.1 color=grey r.colors map=image.2 rast=image.1 r.colors map=image.3 rast=image.1
To write out the color composite to a combined R/G/B raster maps, use r.composite.
g.region raster=lsat7_2002_10 -p d.rgb blue=lsat7_2002_10 green=lsat7_2002_20 red=lsat7_2002_30