You have created or you are creating an Adobe Photoshop file, and you want to change the color of its background: there are several ways to do this depending on the file concerned.
Steps
Method 1 of 4: Change the background color of a new file

Step 1. Open Adobe Photoshop
Its icon is a light blue "Ps" on a dark blue background.

Step 2. Click File
It is, at the top of the screen, the menu furthest to the left of the menu bar.

Step 3. Click New…
This is the first option in the drop-down menu.

Step 4. Click the Background Content menu
It's almost at the bottom of the dialog.

Step 5. Choose a background color
Choose one of the options from the menu.
- Transparent so as not to have a background color.
- White to have a white background.
- Background color to have a background of a predefined color.

Step 6. Give your file a name
Type the name you want in the field name located at the top of the dialog box.

Step 7. Click OK
The button is in the upper right corner of the dialog box. Your file has a new background.
Method 2 of 4: Change the background color of a layer

Step 1. Open Adobe Photoshop
Its icon is a light blue "Ps" on a dark blue background.

Step 2. Open the image that will be edited
To do this, press the two keys Control + O (in Windows) or Command + O (in Mac OS X), click on the desired image file, then on the button To open at the bottom left of the dialog box.

Step 3. Click Window
The menu is in the Photoshop menu bar at the top of the screen.

Step 4. Click Layers
The layers window then appears in the lower left corner of the general window.

Step 5. Click Layer
The menu is in the menu bar at the top of the screen.

Step 6. Click New Fill Layer
The option is in the third part of the menu.

Step 7. Click Solid Color

Step 8. In the dialog box, expand the list of the Color menu

Step 9. Click on a color
Choose the background color that suits you.

Step 10. Click OK

Step 11. Refine the chosen color
Use the Eyedropper tool to rectify the starting color either way (lighter or darker).

Step 12. Click OK

Step 13. Click on the new layer and keep your finger pressed
The layer is in the window Layers down and to the right.

Step 14. Move the new layer
Move it with the mouse so that it ends up above the called layer Background, then release the mouse button.
If the new layer isn't already highlighted, click it

Step 15. Click Layer
Instead, the menu is on the left in the menu bar at the top of the screen.

Step 16. Scroll down the screen
As soon as you see it, click on the button
Merge with the lower layer. It is at the bottom of the menu Layer.
If everything has been done right, the color you have chosen is now in the background of your image
Method 3 of 4: Change the color of the workspace background

Step 1. Open Adobe Photoshop
Its icon is a light blue "Ps" on a dark blue background.

Step 2. Open the image that will be edited
To do this, press the two keys Control + O (in Windows) or Command + O (in Mac OS X), click on the desired image file, then on the button To open at the bottom left of the dialog box.

Step 3. Right click on the workspace (in Windows)
In Mac OS X, you will do Control + Click. This workspace is the dark border around your image.
You may have to zoom out to see the workspace. To do this, do Control + - (on Windows) or Command + - (on Mac OS X)

Step 4. Choose a color
If you are not satisfied with the preset colors, click Select a custom color, choose your color, then click OK.
Method 4 of 4: Change the background color of an image

Step 1. Open Adobe Photoshop
Its icon is a light blue "Ps" on a dark blue background.

Step 2. Open the image that will be edited
To do this, press the two keys Control + O (in Windows) or Command + O (in Mac OS X), then choose the image to modify, finally click on To open in the lower left corner of the dialog box.

Step 3. Click the Quick Selection Tool
This tool, the fourth from the top, is a paintbrush with a dotted circle.
On the toolbar, click on what looks like a magic wand. Hold the mouse button down, a series of tools appear. Click the Quick Selection Tool

Step 4. Place the cursor at the top of the foreground image
Click once on a contour of the part to be cropped, then move the tool: the part will be automatically selected.
- If your image is made up of many details, it is better to work in small parts rather than moving the whole image.
- Once the selection is complete, the tool switches to Add to selection. To extend a selection, drag the tool over the areas to add.
- Continue in this way until the outline of the foreground image is dotted.
- If you have, what happens, exceeded the image at the time of the selection, click, in the options bar of the quick selection, on the right icon of the part Addition. Then click on the area to be subtracted.

Step 5. Click Improve Outline
The button is at the top of the window.

Step 6. Check the Dynamic radius box
It is located in the section Edge detection of the dialog box.

Step 7. Move the cursor to the right or left
At the same time, see what it looks like in the picture.
- When you are happy with the result, click OK.

Step 8. Right click on the background of the image
On Mac, Control-click. A contextual menu appears.

Step 9. Click Swap
This is the second option on the menu.

Step 10. Click Layer
The menu is in the Photoshop menu bar at the top of the screen.

Step 11. Click New Fill Layer
The option is in the third part of the menu.

Step 12. Click Solid Color

Step 13. In the dialog box, expand the list from the Color menu

Step 14. Click on a color
Select the color you want to fill the background with.

Step 15. Click OK

Step 16. Refine the chosen color
Use the Eyedropper tool to rectify the starting color either way (lighter or darker).

Step 17. Click OK
If everything has been done right, the color you have chosen is now in the background of your image.
- To save your changes, click File in the general bar, then on Save Where Save as….