Both spellings are correct. Usually the difference is the publication’s country of origin. In America, the word is spelled “color” and in Britain, it’s “colour.” You’ll find many spelling differences of this variety, such as “harbor” versus “harbour” and “flavor” versus “flavour.” To add or drop the “u” isn’t the only concern. There are many spelling differences between the two versions of English, including “liter” and “litre,” “offense” and “offence,” and “canceled” and “cancelled.”
Some of these changes trace back to early America, when Americans were working toward autonomy and, as part of this effort, created an American English that simplified some spellings. It’s important to remember, too, that language is not static. It grows and changes over time. The creation of a new word—and its spelling—isn’t always straightforward. Take the American “aluminum” and the British “aluminium.” In America, the name of the element was recorded with the spelling given by the chemist who discovered it. In Britain, the chemist’s spelling was changed to conform with the endings of other elements.
All this boils down to a simple plan of action for you, the writer: use the spelling that is appropriate to your audience.