I learned this the other day and thought I would share because I'm having a blast creating new colors for my favorite objects. FireAlpaca is a free program you can download, so no need for photoshop or even layers.
1. Save the image you want to recolor to your computer, open it in FireAlpaca. We're starting with this one for our example:
2. Choose the SelectPen tool and highlight the areas you want to recolor.

The highlight will appear red, you're not coloring anything, just selecting an area.

3. Clean up the edges of the highlight with the SelectEraser tool. I had highlighted the tooth but I cleaned it up as you can see.

You can see the highlighted area better by swapping between any normal tool and the Select tools. That will reveal the area you highlighted while turning the non-highlighted areas blue. You even get the "marching ants" to show you where exactly the borders lie.

4. Once you're happy with your highlighted area, press Ctrl + U or go to Filter -> Hue.
5. Play with the sliders until you get your desired color. Press OK to apply it.
You're done, have fun! Alternatively you can go without the Select Tools, but they allow for more customization options by allowing you to select a small area instead of the entire image. You can also invert the selection by going to Select -> Inverse. I've done that with the above to change the blueish metal to a more silver tone while keeping our gems a bright green.

This can be done with any images really, such as familiars and apparel (though those are bigger and may require more time)
1. Save the image you want to recolor to your computer, open it in FireAlpaca. We're starting with this one for our example:

2. Choose the SelectPen tool and highlight the areas you want to recolor.

The highlight will appear red, you're not coloring anything, just selecting an area.

3. Clean up the edges of the highlight with the SelectEraser tool. I had highlighted the tooth but I cleaned it up as you can see.

You can see the highlighted area better by swapping between any normal tool and the Select tools. That will reveal the area you highlighted while turning the non-highlighted areas blue. You even get the "marching ants" to show you where exactly the borders lie.

4. Once you're happy with your highlighted area, press Ctrl + U or go to Filter -> Hue.

5. Play with the sliders until you get your desired color. Press OK to apply it.

You're done, have fun! Alternatively you can go without the Select Tools, but they allow for more customization options by allowing you to select a small area instead of the entire image. You can also invert the selection by going to Select -> Inverse. I've done that with the above to change the blueish metal to a more silver tone while keeping our gems a bright green.

This can be done with any images really, such as familiars and apparel (though those are bigger and may require more time)