The color of the background of the red stamp may have faded over the years. As for the green one, perhaps it is a cut-out from an envelope? Not sure whether there were any printed in green. -jmh
The green Napoleon stamp looks like a forgery to me. Here are descriptions of Genuine and Forged: FORGED STAMPS OF FRANCE -Paul
I don't think you're doing anything wrong. I expect that the Middle Congo stamp is a variety that isn't listed in standard catalogues. I'm not familiar with these stamps, but as you say, the 65-centime stamp should be on green-tinted paper. Here's a complete set that I found on line: Bob