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I'd like to replace the blue in the linked image with red. What is the easiest and best-looking way to do this? I had the idea of selecting the gold trim and colorizing the inverse, but that would take way too long to trace all that gold.

I can take suggestions for Paint.NET, GIMP, or Paint Shop Pro 9.

Image can be found here: https://www.gettyimages.in/detail/illustration/antique-book-cover-royalty-free-illustration/180820456

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Disclaimer: I'm a Photoshop user, and I know you're not asking for a solution in that software. I do suspect that apps like GIMP have very similar options with similar results.

Put a completely red layer on top of your image layer. (I used a red that was somewhat lighter and a lot more vivid than the eventual effect I'd like to achieve.) Then change that layer's blending mode to Screen or Lighten. Play around with the exact opacity to tweak the effect to your liking.

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    Luckily my copy of Paint Shop Pro has this feature as well. This worked perfectly. – user3163495 Mar 15 at 22:40
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    This also works in GIMP using "Screen" as the blending mode. – Billy Kerr Mar 16 at 10:22
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I agree with the OP's opinion that selecting the gold trim is an interesting solution. We want to take the blue components in the original image and turn them red without changing the blue components in the trim.

Here is an approach that uses GIMP's layer masks and color channel extraction to separate the gold trim from the rest of the image so we can process the non-trim parts independently. Maybe it's more than you need, but my curiosity got the better of me. Since I went through the process already, might as well share!

Using the blue channel to add texture using Multiply blending

I noticed the green channel in the original image could serve as a decent mask for the trim so let's start by extracting it:

  • Create a copy of the original image on a new layer. Name this layer "TRIM MASK".
  • Use Colors/Components/Extract Component on this new layer to extract the "RGB Red" channel.
  • Select this entire layer and copy it to the clipboard
  • Hide this layer, we are only using it to create a mask.

Now we have a decent mask to separate our image. Let's create a layer of gold trim.

  • Create a copy of the original image on a new layer. Call this layer "GOLD TRIM".
  • Right click on this layer and select "Add layer mask". Initialize to black (we will change the layer masks in the next step).
  • Select the layer mask (the black rectangle next to the layer's preview), paste the red channel we copied earlier, and anchor it
  • Hide the original image

Now, we have a layer that is relatively opaque where the gold trim is, and transparent elsewhere.

Gold trim with transparency

You can then add a layer of a flat color underneath. Call this layer "BACKGROUND".

Gold trim layer over flat dark red layer

The flat color looks a bit artificial, so let's bring some of the original texture back in by extracting it from the original:

  • Create a copy of the original image on a new layer between the flat color and the gold trim layers. Call this "TEXTURE 1"
  • Use Colors/Components/Extract Component on this new layer to extract the "RGB Blue" channel.
  • Set the channel blending to Grain Extract
  • Play with the layer Opacity until you get a satisfying result.

Using the blue channel to add texture with Grain Extract

Finally, for fun, I stumbled on this result that I really like by adding the following layer:

  • Duplicate the "GOLD TRIM" layer and place the duplicate just underneath it. Name this duplicate "TEXTURE 2"
  • Use Colors/Components/Extract Component on this new layer to extract the "RGB Blue" channel.
  • Select the layer mask and invert it using Colors/Invert
  • Set the layer's blending mode to Multiply.

Using the blue channel to add texture using Multiply blending

I hope this helps!

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    +1 This may take a little more effort than my suggestion, but I think the result looks really nice! – WaterMolecule Mar 16 at 19:04
  • Thanks! I finally got the hang of layer masks not so long ago and I really like what they allow you to do. But I agree that it doesn't address the "easiest" portion of the OP's question. If anything, any of the other replies can be combined with the GOLD TRIM step above just to preserve the colors of the gold part. Have fun! – – BareMetalCoder Mar 16 at 19:55
  • Way more thorough than I'd ever thought of, +1. Very possibly way more than the OP needed, but awesome to see this process. Welcome to GDSE! Feel free to have a look at the help center if that's needed, and know that there's usually also someone present in our chat if you have questions. – Vincent Mar 17 at 12:37
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GIMP's Rotate Colors

I think Rotate Colors is a good tool for this job. Open the image in GIMP and choose Colors -> Map -> Rotate Colors...

Then choose a blue slice in "From". I chose 184.8° to 273.1°. Then choose a red slice in "To". I chose 327.2° to 47.4°.

Using GIMP Rotate Colors

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I don't use those tools, but I know how we can try it on Photoshop. You might find similar options in those tools you use. One way of doing this is adjust hue saturation. Add a Hue Saturation adjustment layer for this photo, and adjust values for Blue and Cyan.

enter image description here

You should get a required result. It might affect the gold color a bit but I guess it won't matter much. And if you have a high resolution image, it can work even better.

You can alter the values according to the type of red color you need.


Method 2: I just figured it out how we can use another approach using GIMP.

There's Gradient Map feature in GIMP. You can create a custom gradient in gradients window and then apply gradient map using Colors > Map > Gradient Map which can give interesting result. Just make sure you pick brightest colors (which you don't want to change) almost similar to the original image's brightest colors.

enter image description here

Result after using this gradient for gradient map:

enter image description here

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    GIMP also has a similar hue-saturation adjustment. The interface is completely different, but the functionality is the same. – Billy Kerr Mar 16 at 10:54
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    @BillyKerr I like the gradient map much better now, if it is used keeping in mind the attention to details. – Vikas Mar 17 at 12:35

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