There are a number of ways to color-map narrowband astro images. Personally I find Photoshop to be the easiest and quickest way. In this video I demonstrate what I think of as a universal approach to color mapping. It’s also one of the main approaches used by the scientist-artists who prepare images from space telescopes like the James Webb and Hubble.
I encourage you to download the images here (click on each image and right-click to your computer). Then simply follow along.