Rainbow Candy

From level 306 in Candy Crush Saga you will encounter the colour changing candies (changelings!)

You may have seen them earlier when mystery candies transform.

But this is where they become a real part of the game.

The main thing to remember is that they only change between 2 colours, so you can plan your moves ahead once you see which two colours they are changing to.

Here is a video which shows how they change.

Watch the candies with the rainbow flashing over them between moves, these are the ones which will change.

I haven't yet worked out if there is any particular colour that each colour alternates with, but I am watching to see if that is the case.

Watch the rainbow candies for the first couple of moves to see the colours, then you just need to plan 2 moves ahead.

Not too bad once you get the hang of it!
Some people find the colour changing candies in Candy Crush Saga a problem because of the flashing. 
If they bother you too much you can try adjusting the colour or brightness of your monitor until the rainbow candies are more easy on the eye.