All FITS images have a header with info about the image and it contains info about the Bayer pattern as well. This is the order of the R, G and B pixels in the sensor which is needed for Nebulosity and APP (and other software) to decode the B/W image into colour. It probably is RGGB (because this is used by most sensors) but it may be different.
In APP got to tab 0 (RAW/FITS) in the dropbox next to Pattern select RGGB (or the pattern that you can find in the FITS header) and tick Force Bayer CFA. When you then process your images again they should come out in colour.
HTH, Wouter