Nymphs or baby bedbugs are nearly colourless almost translucent when they first hatch and become brownish as they mature. Immature bed bugs maintain their reddish colour for as long as they have remnants of their blood meal within their gut. As the blood meal is digested over time they become more and more translucent again.
Adult Bed Bugs
Unfed adult bed bugs are mahogany to rusty brown in color. Engorged bed bugs are red–brown colour after a blood meal. As you can see in photo A, this bed bug has not fed on a host yet. In photo B the bed bug is fully engorged. Notice how different the shape of the body is.