Re: cold weather = richer exhaust?
By "richer mix" -- you mean the pipe color is blacker? You cannot go by color any longer - not since 1980!
You have a cat converter, it burns the rest of the HC that the engine missed - it changes the color to blacker - soot.
And, in winter, especially in the colder areas, the addatives in the gas change - those winter addatives burn with more soot too - blacker.
There's only one way to know if it's richer or leaner - use a modern exhaust gas analyzer like they use for smog testing. And, you must probe ahead of the cat to see the real thing.
A smog shop will probably charge you about $10-20 for a reading - and they'll explain it to you.
The toys like JC Whitney sell won't tell you anything.