It's a colony even if it's legally part of France.
(Had to check: free movement does apply to Guianans, but in the other direction it's not part of Schengen)
Guianans tend to get pissed off if outsiders suggest in any way that they are any different as a French region from l'Hexagone (the French mainland). Your banging on about colonies would not be well received there.
Ireland is not part of Schengen as well for example, it is because that is much harder to practically achieve for folks living on islands.