First select Photoshop’s Paint Bucket Tool from the Tools palette. By default, it’s hiding behind the Gradient Tool so you’ll need to click and hold on the Gradient Tool for a moment until a fly-out menu appears then select the Paint Bucket Tool from the fly-out menu
Next, click on the Foreground color swatch in the Tools palette.
This brings up Photoshop’s Color Picker. Select whaterver color you want to use for the pasteboard. I’ll choose a shade of red.
Click OK once you’ve chosen a color to exit out of the Color Picker. Move your mouse cursor into the pasteboard area hold down your Shift key and click on the pasteboard. Instantly the pasteboard changes to the new color.