There is an option to show question marks for contacts who are not in Contacts, it's in the app's settings under Contacts.
It should be noted that Gmail appears to save everyone who you reply to in web mail, and then we have Google+ pulling lots of people in.
The color is chosen randomly, and will stay the same over time. As for the letter, well, yes, it's the first letter of the contact's name if specified, or else the first letter of the email address.
Now the important part:
This version has a critical bug in new code, it crashes when going Back from a global search (search in account list: which looks in all loaded messages in all accounts / folders).
I'll be posting a new version with a fix within an hour, as a new topic in this section of the forum.