John Green writes:
I don't face any problem reading 32 bits per pixel files for example
because it is a standard color depth, any developer involved in
imaging knows exactly how to deal with it ( just read the 4 bytes,
extract the 3 bytes related to color: R,G,B and feed them into the
bitmap )

If you do that with a 32 bit/pixel FITS image you'll get a display, but
definitely not a meaningful one ;-) Integer FITS data are actually fixed
point numbers, not integers. The physical value is obtained by
float=BZERO+BSCALE*integer


But a -32 bit (32 bit floating point) color depth is something I have
never heard of. Simply I don't know how to extract the 3 color bytes
(R,G,B) from the floating point data.

To cut it short: I have just one specific problem: how to extract
R,G,B data from the -32 bits per pixel FITS images.

Please read again what has been said to you, and try to understand it.
To cut it short: FITS IMAGES DO NOT CONTAIN RGB DATA in any hypothesis-free
form.