Have you seen The Wizard of Oz lately? The film starts out in black and white. Then when Dorothy opens the door in Oz, the movie suddenly switches to full color. Why not try the same effect in a presentation?
To see how this would work, run PowerPoint and open a blank slide. Choose Insert>Picture>ClipArt and insert any picture. Now, choose Insert>Duplicate Slide to copy the slide. Go back to the first slide now, and select the picture by clicking it. When the Picture toolbar opens, click the Image Control button (second from the left) and choose Grayscale.
Now, choose Slide Show>View Show. When the first slide appears, click the mouse to move to the second slide. The ClipArt picture remains in place, but suddenly appears in color.