I use GIS ARC 9.0 to export tiff image to RGB and then use the index (GNDVI) which is (G-R)/(G+R). I found this index in a research paper. Do you think i can go ahead ?
I guess that you will get the most reliable answer if you describe your needs a little bit more. Image analysis is a vast domain with many dfferent tools and approaches that should be carefully chosen according to the needs.
There are various indexes you could use, but still the question is what problem you wish to solve?