I frequently get asked about how to combine three mono images into one RGB using GIMP. This article addresses how to do just that.

I assume you have three mono images, one for each filter of red, blue, and green.

Note On Update

There is a better approach that you can use, that doesn’t involve installing any software on your system and provides a better result. The details are in this article: Using Photopea.com To Combine Three Mono Images Into One RGB.

Sample Observation

Here are three images, one for each filter, of NGC 1514 – right-click each image to download it and follow along in this tutorial.

This is the blue image:

This is the red image:

This is the green image:

Right-click each image and download it, then follow the steps in this tutorial.

Creating a Combined RGB Image Using GIMP

1. Start GIMP and open all of the files as layers, as shown:

2. Arrange the layers so that they are in order, from top to bottom, ordered in Blue, Red, Green.

3. Set the image mode to RGB as shown:

4. Colorize each layer using the following directions

a. Select the Blue layer

b. Select Color – Colorize

c. Click the colored box next to ‘Color’:

c. Fill in the dialog box as shown (enter 0 for Red, 0 for Green, and 100 for Blue)

d. Click Ok two times

e. Repeat the above steps for the Red and Green layers, noting the numbers to fill in the boxes as shown in the following screenshots:

Colorize box for Red (enter 100 for Red, 0 for Green, and 0 for Blue) :

Colorize box for Green (enter 0 for Red, 100 for Green, and 0 for Blue) :

5. Set the blending mode for each layer to Screen, as shown:

(Find where it says Mode: Normal above the layers, and click the small arrow next to the word Normal and then select Screen)

You now have your combined RGB image:

From here you could combine all of the layers into a single image (select Image – Flatten Image) and then perform adjustments as necessary.

Conclusion

In this tutorial, you learned how to combine three monochrome images into a single RGB image using GIMP.