Narrowband astrophotography uses astronomical filters to capture images of light from specific wavelength bands. It is often used to produce images of nebulae. A dedicated astronomy camera tends to be used.
The resulting frames are monochromatic but can be processed by the Astrophotography Stack Persona just like full-color frames.
Several mono images, each taken with a different filter, can be manually composited into a full-color image.
The most commonly used filters detect Hydrogen-alpha (Ha), Oxygen-III (O-III) and Sulfur-II (SII).
To ensure you start with a document of the correct properties, including resolution and color format, use one of your already-stacked monochromatic documents as the base on which to perform the following procedure.
Copy and paste a flattened version of each of the other two mono images onto a separate pixel layer in the new document.