Fill and gradients
A shape's fill can be one colour, a linear or radial gradient, or nothing. The colour window edits them all, and while it edits a gradient you can drag the gradient's ends on the canvas.
Step by step
Select the beam
Click the beam of light under the lamp. The inspector shows its Fill & stroke section.
Open the colour window
In Fill & stroke, click the swatch beside Fill. The colour window opens beside it: a colour area, hue and opacity sliders, and Hex and R G B A fields.
Make it a gradient
Click Linear gradient in the window's top row (None, Solid, Linear, Radial). The fill now fades from its colour to clear, and a bar of stops appears: click a stop to change its colour, or click the bar to add one.
Point it down
While the window is open, the beam shows the gradient's two ends. Drag the hollow start to the top of the beam and the filled end to the bottom: the light now fades as it falls.
Fade the fill alone
Press Esc to close the window: all you changed in it is one undo step. Then type 70 in the Opacity row under Fill and press Enter. This opacity is the fill's own; the stroke has its own, and so does the whole layer, in Appearance.
Keys
| I | Pick the fill colour from the screen (in browsers with an eyedropper) |