First, make sure that the subject you wish to place within the composition is in the right perspective. Now you need to mask your object. Masking has many methods, you can find a more detailed description about masking here: How to Mask Hair in Photoshop Using Color Range
For this image, I just made my selection with the quick selection tool and refined the edges with the refine edge tool, and finished it with going over hazy parts with a brush in overlay blending mode.
Turn the composition black and white with the black and white adjustment layer. This way you can better see how your subject matches the background.
Now create a levels adjustment layer. Open the “Info” window, by going to Window>Info. With this window open select the eyedropper tool, hold down alt and first click on the darkest part of the background, then click on the darkest part of your subject. Click on the eyedropper icon within the info layer and select Greyscale for both points. This will show you exactly how dark those points are, and with the help of the levels adjustment layer match your subject to the background. Do the same with the brightest points as well. This is how your values should match.
You need to clip this adjustment layer so it only affects your subject (hover between the layers press alt and click) Your layers should look like the example above.
In my case, the cat was too dark, so I brightened it just a little(until the info values matched)
First, create a selective color adjustment layer. Here you need to select all the colors, one by one, and set the bottom bar called “Black” to -100%. For the bottom three “color” from the Colors list called Whites, Neutrals and Blacks the Black slider should be on +100% This turns everything black and ugly, but don’t worry, we made this layer so we can better see the differences between the background and our subject, and will turn it off after you are finished with adjusting the saturation
Now create a Hue/Saturation adjustment layer and clip it(by alt clicking between the layers) to the subject layer. Your layers should look like this:
Click on that little hand, and now click and drag the parts your subject which seems to be off, compared to the background.
In this example, the squirrel had much more contrast, and parts of the cat, at the front legs were oddly grey. By clicking and dragging on them I adjusted these parts.
At this point, your subject may still have a strange color to it which doesn’t match the rest of the image. To better see the differences we need to create two layers. First create a 50% grey solid color layer, and set the blending mode to Luminosity. Now create a vibrance adjustment layer and set the saturation to the max. Now that we can see the differences in colors, as funky as they might be at this point we need to create a curves adjustment layer. This layer should be clipped to the rest(just above the hue/Saturation layer) In this layer once again click on the little hand icon. Now comes the tricky part. As you can see below on the left my subject the cat has a lot of green, and almost no blue in it, unlike the little squirrel next to it.
Inside the curves layer you can adjust the colors separately, Red, Green, Blue, and there’s RGB on top.
While adjusting you need to select the opposite colors. For example, I want more blue. The opposite of blue is yellow, so I click and drag on the yellow part. I did this until the cat matches the rodent more.
There are three opposite color pairs here: Red-Cyan, Green-Magenta, and Blue-Yellow
This last step can be a bit complicated, it’s ok to fiddle around until you get the hang of it.
Don’t forget to disable all the adjustment layers which were meant for highlighting the differences(Vibrance, Grey Fill with luminosity, selective color etc..)
Here you can see how this last step made the green tint disappear.
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