Who decided that republican states would be Red and democratic states
would be Blue, as opposed to Green or Yellow? How is it that all the
news channels agreed on the same color scheme instead of each picking
their own standard?
Excerpt from http://en.wikipedia.org/wiki/Red_state-blue_state_divide
"The United States has a dominant two-party system. The national colors of red and blue have traditionally been used to represent the major parties in U.S. elections. Prior to 2000, there was no convention among media outlets as to which party would be represented with red and which with blue, and though there was some continuity from one election to the next, there was no definitive pattern across the media.
By coincidence, most (but not all) of the major U.S. outlets were using red for the Republicans and blue for the Democrats in the elections of 2000. A practice spontaneously emerged of referring to the states by these colors, primarily based on the results of the presidential election, in which a plurality of the state’s popular vote nearly always determines the assignment of all of its electors in the Electoral College. Commentators and ordinary citizens have broadly adopted the practice. Since the 2000 election, media usage has more closely matched the dichotomy of red for Republicans and blue for Democrats, responding to viewer and reader expectation. Depending on various conventions, this association is sometimes seen as ironic, as red is the traditional color for socialist groups, and blue laws are products of cultural conservatism. On the other hand, “blue collar” is the general term for the traditional non-executive jobs of the working class, an important constituency of the Democratic Party, and red may be associated with the generally more pro-military views of the Republican Party."
You’ll find some useful info here.