I always find this choice difficult:

         ?Ampul/Ampulun/Ampulu değiştirmesi zor bir iş değil.

And what if it was a passive verb?

 Ampul değiştirmek zor bir iş değil ---> To change a bulb is not a difficult job.

                                                    Changing a bulb is not a difficult job.

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Ampulu değiştirmesi zor bir iş değil ---> It is not difficult for him to change the bulb.

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Passive:

Ampulun değiştirilmesi zor bir iş değil. ---> Getting the bulb changed is not a difficult job.

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That´s why I thought I have seen them all... So there can´t be a genitive unless the verb is passive?

Good old tunci, thank you.

 There can be genitive without passive ;

Ampulun değişmesi zaman alan bir iş[tir] ---> The bulb´s changing is a job that takes time.

Bir ampulun değişmesi zaman alan bir iş[tir]. ---> A bulb´s changing is a job that takes time.

But then you removed the causative?

 Ahmet´in ampulu değiştirmesi zaman alır. ---> Ahmet´s changing the bulb takes time.

                                                              It takes time for Ahmet to change the bulb

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In other words, in the light of your examples it seems to me that the logical object (ampul) of the infinitive takes genitive only if it is in the syntactic position of the real grammatical subject (that´s what happened when you added passive or deleted causative).

What do you say, could

         Ampulu değiştirmesi zor bir iş değil ---> It is not difficult for him to change the bulb.

be understood as a generic sentence? Does this "he" have to be a specific person?

 If the person you are telling this sentence knows who are you referring to [specific person] , then you dont have to put "Ahmet´in", otherwise you have to put "Ahmet´in".

what about this ;

Ampulun rengini değiştirmesi alışılmadık bir durum. --> The bulb´s changing it´s colour is an unusual thing.

--- We got causitive now..

ampul|un = stem + gen (modifying the next word, renk)

reng|i|n|i = stem + poss sg 3rd + -n- + acc

değiş|tir|me|si = stem + caus + inf + poss sg 3rd

What is the pink part doing there? Does it refer to

1. the preceding parts of the compound (ampulun rengini) or

2. a person, someone, anyone, who cares who?