Crab - brown meat looks grey
Did the shell on the grey-brown one seem heavier or look weathered?  I have had this more with local caught when I was on the coast than commercial crabs, but, I have had occur something which sounds like what you are describing.  When a crab is ready to change its shell, before it loses the old one it will build up a large amount of chemicals in its body for the new shell.  My experience with this was no noticeable smell, but he meet was off color, soft, and very bad tasting.  I have had this occur in the US with both Dungeness and Rock crabs.

If that was it, the crab would not have hurt you, but also was not something you likely would have wanted to eat.  Sadly, there is little way to know ahead of time that you hit the change of shell.  It usually does not seem to occur with commercial crabs as the big schools seem to mostly change shells together and outside of normal harvest.  I used to fish for rocks though, and they were all over the place, so I was constantly tapping shells and tossing back what others thought looked like good crabs because I thought they were at that stage and did not want to waste one.  The heavy/old shell is not always a solid give-away, but at least it is a hint.

Crab is something that is very easy for a person to react to, so to me it is always best to practice, when in doubt, throw it out.  Sad to waste but better than getting sick, regardless of if the crab was turning or was just in an unlucky wrong cycle.