Marco Presi writes:
> If you want to artificially change the colors, I think the best way to
> do it, is to do it in a selective manner:
> 1) duplicate the layer of the original photo
> 2) change the color of the duplicate layer by using the color balance
> tool to reach the sky tonality you want (don't pay attention to want
> happens to the rest of the photo)
> 3) apply a layer mask to the modified layer (hiding all the modified
> layer)
> 4) select a brush with proper dimensions and draw with it over the layer
> mask: this will reveal the modified layer. If you draw only over the sky
> region, you will obtain the sky with your colors, while keeping the rest
> of the image with original colors. You can play with different brushes
> and different brush settings (I found the opacity setting very useful)
> and see how to get the best results

If you get tired of drawing manually on the layer mask to keep only
the sky, there are ways of getting GIMP to select the sky for you.
Basically, you use Decompose to split the image into various aspects
(hue/saturation/value, red/green/blue or sometimes others) then
use one or more of those layers to help you make a layer mask.

There used to be a wonderful tutorial on that technique by Jenny Drake,
but unfortunately the site is no longer online. The Internet Archive
has the text of the tutorial but no inline images, but you can
read the text here:

And Carol Spears wrote a good tutorial based on it which *does*
have images:

If you can forgive a brief commercial note, there will be a couple
of examples of the same technique in the 2nd edition of Beginning
GIMP (expected in late December). Now that I see Jenny's tutorial
is gone, maybe I'll try to find time to put some of it into a web
tutorial ...

    "Beginning GIMP: From Novice to Professional"
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