Dealing with custom fields

Hello everyone
1) I am trying to get to understand how to use custom fields.
Not talking about display for now but only the implementation inside the backoffice (admin panel)

I went on manage an article and added a custom field
example:

key:color value:pink

Then I went to another article and selected the color field and added the value:blue

Now what I woul like to do is when I go on a third article
and select again the color field to have some kind of drop down menu (could be ajax) to display a choice between the values I already have entered (example blue and pink) a bit the way like tags or categories work…

Anyone who could give me a hint on what to do ?

Since if someone enters a typo example plink it would be considered a different color.

2) Is it possible after that to display let’s say all the posts that have a pink color ?

3) I was thinking using some of this fields to also add info that I will only need in the admin section, how is it possible to show only this info to admins ?

Thanks in advance

Cotp