I have searched but couldn't find one (not online tool only want desktop).
would like one with a GUI.
Is actually a desktop application, with a GUI.
besides is my favourite viewer (really fast, light weight and powerful for many things) , it allows what you need:
-Open the image with it
-Hit "i" key, or the button with the "i" in top bar, or top menu image/information.
-See on "number of unique colors" in that pop up. (leave marked allwas in that pop up dialog "autocunt")
That's it, done usually in one second or less.
EDIT: Sorry, you said "list" not count... Well, I'll leave my answer here in case is of some use.