How does Charisma affect your negotiation skills?

Obviously, higher Charisma means more success in conversation dialogues requiring a Charisma check. But exactly how much of an effect does each point have? How does this work against the difficulty levels of each check? Is there an upper limit for this effect?

After having some negotiation issues (pretty much 100% failure, since I usually treat Charisma as a "dump stat") in my first partial play-through, I actually scrapped the game and rolled a new character with 10 Charisma from the start. (My new logic being that Charisma checks are pretty much the only category of things that you'll nearly always have only one shot, at and can't usually work around it with just patience or player skill.) This character hasn't failed a Charisma check yet. She pretty much, literally, always gets her way.

So, I'm thinking CHA 10 means 100% success on Charisma checks of every difficulty. But I'm not sure. And I also would like to know how checks work at lower levels.

There appear to be three basic "levels" of Charisma checks, as indicated by their color in dialogue menus. In order of increasing difficulty, they are:

• Yellow
• Orange
• Red

So my question, broken down into basics is:

• Does CHA 10 actually result in 100% success on all Charisma checks (with the possible exception of some rigged checks - no spoilers if these exist, please)?
• Is there a minimum Charisma level at which each class of Charisma check becomes automatically successful?
• At lower Charisma levels, do the higher-level checks auto-fail or is there still a chance of success? If the latter, how does each Charisma level affect your chances against each level of check?