How do I remove halos around a replaced sky in a black and white photo?

Question

I am looking for a bit of advice from some Photoshop experts out there. I have a photo in which I have replaced the original dull sky (method explained below) with a sky that is a deeper blue and has more cloud cover.

I have performed the replacement using a colour select method, the problem is however I have been left with a white outline around the original buildings in the photograph that is more visible when you do a conversion to black and white.

Example

Method

1. Load original file and then place the blue sky image on a layer above the original image.
2. Perform a select colour range on the original to select the outline of the sky. Adjust so that all the required areas of sky are selected.
3. On the new sky layer add in a layer mask to bring the blue sky through.
4. Perform the conversion to black and white.

Can anyone help to provide a method of reducing the white outline? Perhaps providing an alternative method or adding an additional step to reduce the effect.

Answer 1

I've done a lot of sky replacements. I've used Topaz Remask, and all of Photoshop's built in tools, and it's never simple. There is always some manual touch ups necessary. This is what works for me:

Initial Selection

Color select usually does a good job if the sky is uniform in color. But any quick masking technique is never going to be perfect.

Another way is to use the Blue Channel, since the sky will be very light in the blue channel, and have good contrast. Find the channels palette, select the blue channel, then Ctrl/Cmd click to select it. Then use that selection as your initial mask.

Fine Tune the Selection

Once you have your mask, use the Refine Edge feature to let Photoshop try to fine tune the selection. Play with the hardness and radius sliders.

Next I would use Quick Mask. This displays the mask in pink over your image. Zoom in, and use a black or white brush to adjust the mask. This will be easy around the buildings, but the trees will be to much work.

Feather Selection

Finally, feather the mask by a pixel or two.

Remove any halos

Finally, if you merge your layers, there is a neat trick to remove halos. Let's say you have a tree, which is relatively dark, with lighter sky in the background, which contains an even lighter halo. I think that is the case in your image.

To fix this, use a brush, and pick a good color in the sky near the area you want to fix. Set the blending mode of the brush to Darken. Then paint over the halo. Because the tree/building is darker than the sky, the brush will not affect them at all, and won't affect the sky since the color you're using is the same, it will only darken the halo to match the sky.