The key point is - 'underexposure' causes lost of color information and it is not advisable to go far too low from correct exposure, and certainly not ISO 100 for shots that need ISO3200. Some think one should just stick with in-camera processing.
To understand this better, we plot the histogram/levels curve for the same photos as below:
The correctly exposed ISO 1600 has a smooth distribution curve. The under-exposed ISO100 has a distribution curve concentrated on the left. To get the right photo, we need to 'spread' the distribution out. The typical way is use to gamma correction. I tried it out first but it looks terrible. See the 1st photo below. The curve becomes spikes showing loss of information. However, PhotoBrush has a EV correction function and I tried it with max 4EV and than another 2EV to get the final result show above. The key part (right side) of the curve is quite smooth (due to interpolation when the image is saved) but you can still observe some spikes in the left part of the curve showing worse interpolation at the left end (darker side) of the histogram. This is due to JPEG non-linear tonal curve compression method. The end-result is, in my opinion better than ISO1600 photo - smoother, more contrast and better letters details. But the ISO1600 preserves better texture and to experts, a better photo. But you need to make your own judgment and decide on your photo taking and processing technique.
The histograms of the other two post processing software are shown below. We can see that the Picasa one is made up of more spikes. The FinePix Viewer seems to interpolate quite well but till under-exposed and can be adjusted further.
In summary, when you under-exposed your shots, you will loose color information and hence don't under-expose too much. I believe you have the alternative to the in-camera ISO3200 option with a few stops lower ISO 1600(auto), 800 or 400(auto).
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