Select the gradient tool, then pull down the arrow next to the current gradient at the top and select "new gradient". Name it.


Select your new gradient and then click the gradient up at the top to edit it.


Double click the color / slider controls at the bottom to change the starting and ending colors (and intermediate if you want to add them. Be sure to "Save" your changes.


Now draw your gradient in your picture and bingo!

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http://iit.bloomu.edu/vthc/Photoshop/DRAWING/gradients.htm A little more about Stops.