Morph Bubbles are introduced at level 106. They change colors. This will tell you the possible color combinations:
Red can change to purple or green
Purple can change to red or blue
Green can change to yellow or red
Yellow can change to blue or green
Blue can change to purple or yellow
As there are only two colors that each bubble can change to, if you have no red bubbles then the green has to change to yellow. If you have no green bubbles the red has to change to purple. If you have no purple bubbles the blue has to change to yellow, etc.
But as it is quite complicated, it's much easier to just watch the bubbles to see how they change and to what colour.
See the example below:
You can see in these two images how the dark blue changes to red.