Step 1 – Open Document Image

To begin, copy/paste the following image into Photoshop. Or right click; choose Save Image from your browser context menu. Save it to a location on your computer. In Photoshop, File > Open and browse to the previously saved image to open it.

Once the image is open, click the Add New Layer icon command at the bottom of the Layers Panel (encircled in red) to create ‘Layer 1’ as I have captured below.

Step 2 – Channel Observation

Before doing anything else, switch to the Channels Panel (Window > Channels) and inspect each of them. In this case the Red Channel already contains the dark contrast needed to isolate the dress to black.

Step 3 – Apply Image Command
Switch back to the Layers Panel, and with ‘Layer 1’ active:
Go Image > Apply Image command, and at the commands dialog prompt just change the Channel option from RGB to Red as I have captured below. Then click OK to commit changes.

Step 4 – Change Layer 1 Blend Mode to Darken
Once the Apply Image command is applied, simply change the Blend Mode of Layer 1 to Darken, as captured below.
What was once green (the dress) is now black and done with little effort and without compromising the surrounding color information?

Here is another example, except at the Apply Image step, the Green Channel was the better choice for the final conversion to black.

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Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry’s standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software.