There is a difference between true monochrome and Bayered images (which are grayscale). SGP does not do any debayering. As such all images viewed in SGP will be grayscale. However OSC images will always have color data associated with them. If you open up PI and debayer the images you will indeed get color from a “single channel” OSC image. SGP does not output 3 channel (Red, Green Blue) FITs as they would be 3x the size (and are honestly obnoxious to process…)
If you would prefer to work in NEF rather than FITs, SGP can save off the untouched .NEF files as well. Just choose the NEF option in the Nikon Settings Dialog.
Might help to better understand how “OSC” cameras actually work. The CCD/CMOS itself is a monochrome sensor (technically it reads light levels and not color…but let’s not go there). On top of the sensor is a Bayer Matrix which consists of pixel sized filters. This Bayer Matrix is really the only difference between a monochrome and a OSC camera using the same CMOS/CCD chip. Each pixel has a single color filter overlaid on it. Generally in a block of 4 pixels you’ll generally have 1 blue, 1 red and 2 green. So you take an image and each of those pixels captures light through a single filter. The output is an image that has intensity data for each of those pixels, but no actual color to it (as the sensor is monochrome). However there actually is color information in the image as each pixel has a color filter in front of it.
Debayering is the process of matching the pixel with the correct color filter, then combining near by neighboring pixels to create a RGB image.