I took a look at the code. Orange seems to either display a broader region or an "outside" region (or province, they're called regions in the code).
In Assets/Python/Consts.py there are lists of lists called lNormalRegions and lBroaderRegions (plus others but those don't matter here).
To change a region from orange to green, you'll first have to check if your civilization's list in lBroaderRegions contains that region; if yes, remove it. AFAICT lBroaderRegions isn't shown on the map, but it may cause problems if a region is in several lists of a single civ so it's best to remove it.
Then you'll want to add the region to your civ's list in lNormalRegions, which are the green provinces. And that's it.