After you have set the animation for the bullets (fly in from the left), switch to slide view. Click on the bullets in the slide to select that portion of the slide and then select Custom Animation from the Slide Show menu. Click on the effects tab to bring it forward. Click the pull-down menu under After Animation and select a light gray color. Select OK to close the dialog box and then Animation Preview from the Slide Show menu to view the results. As new bullets fly from the left the color you selected will be used to "dim" old bullets.

Example,