When publishing images online for general consumption, images that have color tend to  get better feedback than images that do not have color. The fact is, however, that you’ll one day end up with a monochrome image that has something of interest in it, yet you want to colorize it to improve its presentation.

This article walks you through colorizing an astronomical monochrome image, to improve the subjet’s appearance, while maintaining the ‘natural’ colors of the stars.

While I use Photopea in this tutorial, you could certainly adjust the process for Photoshop or GIMP.

The Result

I started with an image of HCG 30 that was monochrome:

I labeled each member of the group to highlight it for comparison.

After making adjustments, I ended up with a color image of HCG 30 having the galaxies blue in color while leaving the stars unchanged:

Overview Of The Process

Adding false color to a monochrome image involves simulating the Red, Blue, and Green filters used during image acquisition. We do that by creating copies of the background layer and adding our own adjustment layer that adds the required color (red, green, or blue).

Once we have that, we change the blending mode of the colored images so that they combine to produce a full-color image. From there, we adjust the image’s Curves adjustment to make the galaxies bluer in color and to darken the background.

Before doing all of that, we isolate the stars in the image to retrain their original color and appearance. All we do with the stars is blend them into the resulting image by changing their blend mode to Screen.

I assume you’re using Photopea for this process; however, the directions can be changed to accommodate Photoshop or GIMP.

Sample Download

I include the image from this article for download. The image is a PSD that you can open in Photopea, Photoshop, or GIMP. You can remove the existing layers from it to return the image back to its original monochrome state so that you can follow along.

Selecting Only The Stars

Fortunately, the stars in this image are not among a nebula or other object for this image, so selecting just the stars is relatively easy: just select the background and then select the inverse to select only the stars; I explain how to do that here.

Alternately, if the stars are among the subject, like a nebula, you can still select them – refer to this article about how to do it: Selecting Stars In Your Image And Editing Them

Start by opening a monochrome image in Photopea. If you don’t have one handy, you could tye to use the one from this article.

1. From the menu, choose Select – Color Range

2. Click Replace next to Mode

3. Click a dark area of your image

This captures the background, but we want the inverse, so from the menu choose Select – Inverse

Photopea is good at making selections and makes them exactly as large as they need to be; however, that’s too close for our purposes, so we need to expand our selection to accommodate star halos and other artifacts. From the menu, choose Select – Modify – Expand and enter 2 pixels

Inspect the selection and ensure it covers the stars – modify the selection as necessary.

We have everything selected, except the background and this is not what we want – we want to be able to make changes to the galaxies in the image while leaving the stars alone. We need to deselect the galaxies.

1. Click the selection tool in the toolbar to make it the active tool

2. Press and hold the ALT key on your keyboard

3. Select each of the galaxies in the image – this deselects them as you make your selections

At this point, you have only the stars selected, so ensure the background layer is active and from the menu, choose Edit – Copy.

Now from the menu select Edit – Paste to paste the stars as a new layer. Rename the layer ‘stars’ as a reminder about what’s on it.

Making The Adjustments

1. Start by duplicating the Background layer two times (not the Stars layer), so that you are left with three layers. Rename each layer, starting from the bottom, as ‘green’, ‘red’, and ‘blue’.

2. Activate/highlight the green layer and from the menu, select Layer – New Adjustment Layer –  Photo Filter.

3. For the color swatch, select green

4. Repeat the previous two steps for the red and blue layers, and choose their respective color swatches

5. Select each of the Red and Blue layers and change the blend mode from Normal to Screen (you’ll find the blend mode above the listing of Layers – it normally says ‘Normal’ with a small arrow next to it)

6. Select the photo filter layer for the blue image and add a Curves adjustment layer – select Layer – New Adjustment Layer – Curves

7. Adjust the Curves layer to darken the background by moving the dot in the lower left of the graph slightly to the right so that it is at the base of the graph

8. Add a Hue/Saturation layer above the Curves layer –select Layer – New Adjustment Layer Hue/Saturation

9. Adjust the Saturation to about 40

10. Adjust the Photo Filter intensity for the remaining layers as necessary; avoid reducing the green by much and adjust red and blue to suit your tastes

11. Change the blending mode of the stars layer to Screen

Save Your Work

You now have a final false-color image that you can save or publish. Save as a PSD, or in the native format of the application you’re using, to allow you to edit the image again in the future, should you wish to make changes.

Conclusion

In this tutorial, you learned how to create a false-color image from a monochrome original. Along the way, you learned about making selections, adjustments, and learned about how blending modes affect the final image.