[Q]

It is not necessary that you have a new colony that is angry to test the effect. Just make any colony angry, remove all causes for the anger, don't do anything to improve the happiness and see what happens.
I modified some happiness types and gave them rather high (negative) values for the natural decrease (up to -60).
The effect was that I had most colonies in the indifferent state, but not angry or rioting as you would expect if the effect was the same all over the entire happiness scale.
What you say about the "Natural Decrease for Other Races" makes sense. Apparently the value of 20 is too small to give a clear effect, or my empire management is so good that it compensates more than enough: I have always many foreign races in my empire and they are jubiliant all the time! But as you said if we test it with higher values (and/or negative values) we should see the effect.