Sparky,
Looking back to the '62 NEC, 210-5 says:
Color Code. Where installed in raceways, as aluminum sheathed cable, as open work, or as concealed knob-and-tube work, the conductors of multi-wire branch circuits and two-wire branch circuits connected to the same system shall conform to the following color code. Three-wire circuits - one black, one white, one red; four-wire circuits - one black, one white, one red, one blue; five-wire circuits - one black, one white, one red, one blue, one yellow. Where more than one multi-wire branch circuit is carried through a single raceway the ungrounded conductors of the additional circuit may be of colors other than those specified. All circuit conductors of the same color shall be connected to the same ungrounded feeder conductor throughout the installation.
Abbott's NEC Handbook (Frank Stetka) adds the explanation:
The color coding of multi-wire banch-circuit conductors is necessary in order that the loads may be properly balanced between the ungrounded conductors.
Al
