Imagine you found a really cool photo on the internets and you wanted to generate a colour palette off it to make cool forum posts like me.
1. Get gimp cause photoshop is for scrubs.
2. Get an image.
3. Open the image in gimp.
4. Go to Colours -> Posterise.
5. Tweak the slider until you find a value that you like.
6. Go to Image -> Mode -> Indexed, then in the "Maximum number of colours" put in the number you posterised the image to.
7. Go to Windows -> Dockable dialogues -> Colourmap.
You'll be presented with all the colours in a form of a table. When you click on one of them you'll see its HTML notation. You can use these colours right away or put them into another palette generator tool (like Adobe Color). The palette generated from the image above posterised to 6 colours is:
331c4e 373a37 7a323b 674984 de8088 89bbc5
You can of course posterise to more or fewer colours to get different results.