Cockroach Moult?

When I looked in my roach tank last night I noticed that one of the roaches (Norman) has turned a strange colour.

Here's what Norman used to look like:

                         Norman a few weeks ago.

And here's what he looks like at the moment (Norman on the left):

                                        Norman at the moment.

I took the above picture with Rusty (right) in my hand also so it would be easier to see the colouration. He's turned a strange sort of coffee colour, as opposed to the reddish colour his back used to be. He's also quite a lot bigger (or ahem, chubbier).

This leads me to believe that he may be having a moult. Cockroaches moult throughout their lives, meaning they shed their outer layer or exoskeleton. This happens when the cockroach has 'outgrown' its original exoskeleton and allows the roach to grow further. Here's a picture of a cockroach when its just moulted:

                          Cockroach following moult (link).

They emerge from the old exoskeleton white and soft to the touch (it doesn't really look like the same animal).

So apparently the only other reason a cockroach may colour other than moulting is a change in diet. Though surely this would affect my other four roaches? They are all the same colour they have always been.

I'll keep the blog updated if anything happens!