I didnt pay much attention to the hue of those gems, not really important actually. Let's just say 2 gems with the same colour and same grade but different hues will have a very slight damage output difference.
I've never combined two gems of different colour before, unless i'm using them as a gem bomb of course. Combined gems are also weaker, for example, a grade 2 pure red gem is definitely stronger than a grade 2 red-green one. I dont think there's such a thing as immune? Well, try and use a gem bomb if there is! lol hope that helps.
I've completed the game with a score of 340m, not exactly high compared to some over here. and i'm still trying to figure out how the top scorers managed a score of 2 billion.. But at least you can be sure i'm not a beginner and crapping to you. lol cheers.