Recently I asked for advice on making an effect that will simulate an optical grad filter. This would be for for use when you forgot to put a real one (or a polarizer) on the camera, and you have good exposure on the subject but you've blown sky and clouds. In this situation "curves" can work wonders in bringing out recorded detail left in the sky that is being clipped in the process. At other times you just want to darken or change the color of the sky a bit -- or even create one.

I got some good suggestions from jrazz and others and did some experimenting on my own. I think the quickest and easiest approach that works for me is to use the "linear black to transparent" generated media as an overlay. Rotate it -90 degrees to put the black at the top, and size it appropriately. Move the transition center point, slope, and transition gradient width using the buttons on the graphical indicator. Change the color and saturation with the slider bars.

When you think about it, this is exactly what an ND/color grad filter does on the camera. Before, using "curves" I've been able to recover highlight detail that was recorded but otherwise invisible on a calibrated monitor under default conditions (the curve ended up looking a little wierd). This grad process sometimes makes it look better.

What it does not do is recover detail that has been clipped in the camera, but maybe you can then create some sky.

Gary