You've likely seen those pictures of roses that don't have any color except the red. Here's how you do that:
First, you'll want a picture that has a lot of color so the effect will be noticeable and cool. If there aren't a lot of colors, it will be far less impressive. So, I found a nice patriotic picture I took with reds, greens and blues.
In Gimp, go to Colors->Hue-Saturation...
Select the B (Blue) radial button and drag the Saturation level down to -100:
Notice how the blue is now gone from the flag, but the sky is still blue. What's going on?
Still Some Blue
Well, the sky is actually more Cyan in color than Blue, so you'll need to select the C (Cyan) radial button and also get rid of the saturation for that color as well.
Repeat this with all of the colors except the one you want to keep:
But wait, there's some red in the tree! That's no good. So duplicate your color isolation layer by dragging it to the "Duplicate Layer" icon:
Go to Colors->Desaturate...
And pick one of those options: Ok
Now your whole image is black and white, so select the Pen tool and draw a path (by clicking on various points) around the flags. You don't need to be too careful here because you only need to keep the red parts of the flag.
Again: The pen tool doesn't really draw on your picture. It merely creates a "path" that you can use for several cool things. But one of the most useful is creating custom selections. So, once you've made it around your object, right click and choose:
Right click again to Edit->Cut
Why is there a big while spot? Shouldn't the selected area have gone away?
Yes, it should have. But if this happens to you, right click on the layer in the layer box on the right you want to cut out and select:
Add Alpha Channel
Now when you cut or delete something the Alpha channel will make that part transparent instead of white.
And you're done!
Flags with Isolated Red
Your Media Production Mentor