What is the idiom for someone who is in a good mood or position? For example, your colleague is in a very good mood because of a recent thing that happened to him.
A common expression for that emotion is on top of the world.
In the UK, you’d often hear the colloquial expression, ‘chuffed to bits’ (effingpot.com).
Other options include ‘in seventh heaven’ (thefreedictionary.com), ‘on cloud nine’ (ditto), ‘as happy as ...’ (Various options here (answers.yahoo.com), or make up your own), and adjectives such as ‘ecstatic’.
You can say he's in high spirits. http://www.thefreedictionary.com/in+high+spirits
Someone who's content because they're in a good position as a result of something that [usually, recently] happened might well be described as...
sitting pretty - in a good situation
But it's worth noting there's often a suggestion of envy in the usage. On reflection, I think the vast majority of expressions that simultaneously allude to someone's current happiness and the fact that it was caused by something that [just] happened probably tend to convey some degree of envy (facetiously or not). That's Anglophones for you.
The term chipper would adequately cover this, and is perhaps slightly more formal than some of the (perfectly valid) examples in other answers.
Elated is a neutral word that can be used in any English-speaking country.
"Happy as a Clam" is common in the US as well.
How about "in the catbird seat"? This goes more toward position than mood.