Combining three mono images into one RGB is a common problem. Fortunately, the solution is very straightforward.

This article walks you through combining three mono images, one for each color of red, green, and blue, using Photopea.com. Using Photopea.com is the recommended way to do this because there’s nothing to install on your system and you get great results.

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 Photopea.com

  1. Point your browser to Photopea.com
  2. Get the dimensions of one of the images. From the menu, select File – Open
  3. Select the blue file, NGC1514-B.png
  4. From the menu, select Image – Image Size and note the dimensions of the image (in this case, it is 400 x 400 pixels)
  5. From the menu, select File – New
  6. Enter 400 in the Width and Height boxes and click Create
  7. At the bottom-right, where it says Layers | Channels, click the Channels tab to make it active


  8. Open the remaining two images, NGC1514-R.png and NGC1514-G.png
  9. Click the New Project.psd file at the top of the Photopea.com window


  10. Click the word Blue in the Channels tab


  11. Switch to the blue image at the top of the Photopea.com window
  12. From the menu, select Select – All
  13. From the menu, select Edit – Copy
  14. Switch back to the New Project.psd file with the blue channel active
  15. From the menu, select Edit – Paste

Repeat from step 10 for each color image, and select the corresponding channel in New Project.psd. When you are finished, click the eye icon next to RGB to reveal the combined image.

This is the resulting image:

Save your image by selecting File – Save as PSD in native Photopea/Photoshop format, or select File – Export As and select a format to export your file.

Conclusion

In this tutorial, you learned how to combine three mono images into one RGB image using Photopea.com.