Many years ago, when I was shooting Kodachrome slide film, I got into the habit of underexposing my pictures a bit to bring out the colors. I also used a polarizing filter quite often to enhance color. However, this approach usually led to an unrealistic sky. The sky would sometimes turn such a dark blue that it almost looked like a night sky.
In the digital age, this wasn't so problematic. Whenever shooting a clear blue sky, particularly at high elevations, I remove the polarizer and then adjust the tone and color of the sky to my liking during post processing.
During my last road trip, I was faced with a different challenge. I was shooting some old rusty cars at the Bodie Ghost Town in the high Sierras. I wanted to use my polarizer to bring out more color in the cars, grass and buildings (by minimizing the glare from the bright sunlight). But, from past experience I knew that the polarizing filter would destroy the sky by turning it practically black.
Since I've been experimenting with so many different blending techniques in Photoshop lately, I came up with this solution:
I took two pictures of this scene. The first was shot without a filter. The second was fully polarized.
Of course, both pictures were taken with the camera secured to a tripod to eliminate any movement between exposures.
Then, using Photoshop I was able to take the best parts of both pictures.
I placed the fully polarized picture in a separate layer on top of the first photo. Then, I masked out the sky (painted it out in black) so that everything except the sky is polarized.
The effect might seem subtle, but this approach enabled me to apply a filter effect selectively within a scene to enhance the colors and bring out more contrast right where it was needed.
Here's the finished product: