If you like the look of a rainbow gradient but can't imagine trying to manually assemble the array of colored vertical stripes required, rely on Photoshop's quick gradient-making tool. Gradients in Adobe Photoshop are well-suited for image backgrounds or to serve as the entire image themselves, with perfectly proportioned colors you can customize.
Open Adobe Photoshop. Pull down the "File" menu. Select "New." Type "RainbowGradient" into the "Name" box. Type dimensions into the "Width" and "Height" boxes for the gradient canvas size, such as "4" for each for a square gradient. Click "inches" from the two drop-down menus. Click the "OK" button.
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Click the "Gradient" tool, which looks like a rectangle with a black to white gradient, in the middle of the "Tools" pane. If the tool is not showing, right-click the "Paint Bucket" tool. Choose "Gradient Tool" from the fly-out menu.
Review the gradient toolbar at the top of the screen. A rainbow gradient may have already defaulted. If not, pull down the gradient menu and choose one of the rainbow gradients.
Click the first of the five small icons to the right of the gradient menu. This is the icon for the traditional vertically striped gradient.
Position the cursor on the left side of the "RainbowGradient" canvas. Draw a line to the right side. Don't worry if your line extends into the gray space of the Photoshop work area; it won't affect your gradient. The rainbow gradient fills the canvas.
Experiment with different gradient styles by continuing to draw lines, such as top to bottom or corner to corner. The rainbow gradient remains, but changes direction. Try one of the other four small gradient icons, which keep the rainbow but give you gradients in other shapes, such as a starburst or diamond.