Learn English – Use of the word “have”

You're correct that "o" is US and "ou" is non-US. It'd be considered bad style to switch between them in the same text. Generally, you should just choose one style and use it consistently, and you will be understood. I've heard a rule that if you're writing for a mostly American audience, you should use the American spelling, and otherwise use the international forms, but that may not even be necessary.

One place that mixing styles is allowed is when quoting verbatim from text, or in technical literature where spellings must be retained exactly:

I asked him what colour he wanted, and he said "I'm no good at picking colors".

The color: #ffffff; property indicates a text colour of white.

"I have never said this" is in the present perfect tense and implies that at no time in the past have you ever made the utterance that "this" refers to. You might say this in response to an accusation that's shocking to some degree: "You told some of my friends that you hated me!", "No, you're wrong, I have never said that".

"I never said this" is in the simple past and implies that in one particular instance that is understood by the context you did not say what you're being accused of saying. "Did you tell Dale that you were going to wait for me on Tuesday?", "No, I never said that".

Note that in both constructions, it's probably more usual to use the demonstrative adjective "that" rather than "this" -- why? Because it seems to me that these phrases would most likely get used in response to a particular point by someone else said previously, and so you want to refer to that particular point.

If someone had a written phrase on paper, pushed it across the table to you, and then said "we believe that you made this statement", then you might look at it and say "No, I never said this" where "this" indicates "what's written on this paper".