While there are ways to do this in Photoshop itself, the best results come from using Camera Raw’s HSL controls (I know, I sound like a broken record). From the Filter menu choose Camera Raw Filter (or, ideally, do this to your RAW image in Camera RAW before you open it in Photoshop as an 8-bit or 16-bit image). Then, click on the HSL/Grayscale icon (it’s the fourth one from the left) beneath the histogram. Click on the Luminance tab (that’s what the “L” in HSL stands for—Luminance), and then simply drag the Blues slider to the left and your dreary sky turns bluer. If that didn’t get you enough blue (or you just didn’t like the brand of blue it gives), double-click on the slider to reset it to zero, and then click on the Basic icon (the first icon on the left, beneath the histogram). Drag the Vibrance slider to the right to make any blues in the sky much more vibrant. Okay, lastly, if there isn’t any blue in your sky, try this: click on the Adjustment Brush (K; it’s the sixth tool from the right) in the toolbar, then in the panel on the right, scroll down to Color, and click once on the white box to its right (it has an X through it, indicating that no color has been chosen). Clicking this brings up the Color Picker. Choose the blue color you want for your sky (actually, choose a little bit darker blue than you think you’ll want because you’ll only get a tint of it), then click OK to set your color to that blue. Now, just paint right over your sky in that blue (as shown above). When you’re done, click on that blue color swatch to bring the Color Picker back up, lower the Saturation amount to 0 (that resets the color to “none”), and click OK. Otherwise, that blue stays on all the time.
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