You can extract R,G,B bands from hyperspectral image by extracting bands present in the wavelength range of Red, Green, Blue. For this along with the image (GeoTIFF file) metadata of wavelength range present and spectral resolution (difference between two wavelength bands) is also required.
Say, after reading the image file data is in a cube form of size M*N*K where M is the height, N is the width and K is the range of wavelength bands present. You can get R, G, B bands by
DataCube(:,:,range) where range is the desired wavelength range.
Now, for GeoTIFF images before downloading the image have a look at the wavelength information of the Sensors this may help you with the metadata required.
