Compare the three sample exposures, noting the difference in light and detail. Each filter isolates light from a different ion, so the same nebula appears brighter, darker, sharper or more detailed in different places. Those differences are the gas structure recorded in each wavelength - and they are what become colour when the layers are assigned to red, green and blue.
Try your own images: select Swap photo on each panel and load three filtered exposures (FITS, JPG, or PNG) of the same target. Hα is used as the reference frame, and the other two images are automatically shifted horizontally and vertically to compensate for ordinary tracking drift before the composite is built.
BASIC ALIGNMENT ONLY - use images with the same orientation, scale and approximate framing. Rotation, flipping, resizing and major framing differences are not corrected. FITS, jpg, or png files accepted.
Use the white marker on each spectrum strip to see the wavelength that filter isolates. The black point, white point, gamma and strength controls change how strongly each greyscale layer contributes to the final image.
Pick which exposure feeds Red, Green and Blue. The same exposure can feed more than one channel - that's how "bicolour" palettes fake a natural-looking image from only two filters.
OIII often dominates wherever Hα and SII are both weak, giving a green cast no astrophotographer wants. SCNR clamps green to the average of red and blue, pixel by pixel.