I have provided some infomation about nouns for you:
Proper noun - used to refer to name a person, place or thing. It begins with a capital letter.
Common noun - is a noun used to refer to a person, place or thing in general such as colleagues, all. It is usually written as a capital letter at the beginning of the sentence only.
When using colleagues in a title you would use a capital letter such as Dear Colleagues.
Hope this helps
(Student teacher so I am still learning)