In the constructed language community, the color argument is an often repeated argument that an international auxiliary language based on the languages of one area is only suitable for the inhabitants of that area. Whether or not it is true is often a subject of heated debate.
The term itself comes from the phrase "as European as the color green", which was used in the defense of Esperanto during the early nineties on a constructed language mailing list. The intention was to note that Esperanto's vocabulary list, despite being of European origins, describe ideas and necessary concepts found everywhere, regardless of linguistic background. The phrase eventually changed into "as biased as the color green", and stuck. (See distinguishing blue from green in language.)
Nowadays, the term color argument is used to refer to a discussion about constructed languages that centers solely on the sources of the vocabulary and/or grammar, as opposed to any other merits or flaws of the language.