If you look at one closely, you'll see that the borders of the button are of a different color than the field, which gives the illusion that light is hitting the button from the upper left hand corner of the window. When folks started using color in the interface, someone came up with the idea of using color to make the button look like it was three dimensional on the screen. The trick is to use a gradient . A gradient is a ramp of colors, which appears to smear and blend the colors between one point to the next. In or example, the gradient is vertically oriented going from a light color to a darker color.
Some folks just draw the gradient directly to the screen.
Other folks store the gradient as a bitmap then replicate it as needed. It's the age old computer trade off, speed versus storage. We're going to draw it straight to the screen, because we trust that Mr. Moore is right. Time to whip out our Morphic bag of tricks. We know that we need a gradient of some sort.