On the same night you shoot your images, obtain a set of dark frames, and if possible, make flat field frames also . In an ideal world, you would shoot separate flat fields for each of the colors because the color sensitivity of the chip may vary over its surface. If because of time constraints you can shoot only one flat field, shoot it through the green filter. Calibrate each of the images . If you are using CB245 or Multi245, be sure to use Dark Matching to obtain the lowest possible noise level.
Good calibration is important to avoid sky backgrounds that change in color.
If you have shot multiple frames in each color, use Multi245 to register all of the images to a common star, then average the images taken in each color. This will give you red, green, and blue tricolor frames with very high signal to noise ratios and already in perfect register. For the best color, avoid the non-linear scaling laws . Use stretch or linear scaling even though this usually entails some loss of highlight detail. The color-related functions ask for a saturation parameter.