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.
Generally, famous refers to recognition, and popular refers to reputation (of a person) or frequency (many people use a thing or visit a place).
Famous often, but not necessarily, has positive connotations. Knowledge of the famous person, thing or event is widespread.
Popular usually has positive connotations, or when referring to a place, it means it is visited by many people. The person, place or thing is largely well regarded among the people who know of it, but doesn't necessarily mean a lot of people know of it.
Perhaps it's easier understood with an example.
- A famous restaurant: Is well known. The name of the restaurant is recognized, perhaps because a well-known chef founded it, or it advertises a lot, or a well-known event occurred on the premises.
- A popular restaurant: It's usually busy. The majority of people enjoy eating there. Or maybe they don't, but they go there because the food is cheap, or it's the "place to be." Outside of the people that eat there, it may or may not be well known.
A restaurant can be both famous and popular (everyone knows about the restaurant, and many of them go there), or popular but not famous (only the people that frequent the restaurant know of it), or famous but not popular (making it infamous).
Best Answer
According to the Oxford English Dictionary, this word comes to us from the Latin word color-em, which was inherited by old French. The o segment in Old French was actually somewhere between [o] and [u] and was represented by the digraph ou. When this syllable became accented, it became fronted, represented by eu (this the Modern French couleur). The corresponding English word of old is the ancestor of hue.
The world was adopted directly into Middle English as colur; later colour from the Old French color, culur, colur; later colour; and finally coulour, which was the preferred Anglo-French spelling. Colour was eventually standardized in post Anglo-French English. Noah Webster's 1828 dictionary is credited with standardizing the latinate color in American English.