There are certainly many ways in Photoshop to edit and match colors within a scene, but here is a quick tip that will work with primarily monochrome scenes such as the one displayed above. The mermaid was shot in a studio setting and then composite the image into the coral scenery. The overall cast of the scene is sort of a hazy blue with some of the corel being slightly darker. Obviously we want to at least match the mermaid to her surroundings as close as possible.
There are numerous tutorials on compositing, and I have discussed different techniques in the past such as using greenscreen or even shooting on a grey background. For this quick tip, lets assume that you have your subject placed and sized appropriately on the background with at least an appoximate brightness that you desire. Now take your "eye dropper' tool, and sample a clean area in the background. In the image above, I sampled away from the corel in the top portion of the image to get a 'True blue" sample.
Now, take a VERY large paint brush, set to a opacity of 20% or less. This setting may be slightly more or less depending upon your image. Paint over the entire image. You may have to take more than one pass over the entire image to help with the process. What you should find is the tone of the image including your composited portion now shares a similar appearance.
Will this technique work for every image, certainly not images that have a lot of vibrant colors. Again this technique is one way you may get a composited monochrome image to look a little more natural. Other tools within photoshop that may help include the 'blend-if" functions, hue/saturation layers as well as using trying the 'color match' feature of photoshop. Photoshop has a myriad of tools, and almost certainly one will work for you.
Bob Barford is a photographer located in Sourthern PA.