A battery maintains the highest electric potential at its positive terminal. Any wire attached directly to this positive terminal will have the highest possible pressure and be colored RED. The color red should be used to color any wire from the positive terminal out to the first light bulb shown at the top of the diagram.
Similarly, the negative terminal of the battery is the lowest possible potential. Any wire connected directly to this terminal is assigned the color blue. Blue should be used to color any wire connected to the negative terminal out to the last light bulb shown along the bottom of the diagram. (In case you're wondering, the positive terminal is represented by the longer line at the top of the battery circuit symbol.)
Charge flowing from the positive to the negative terminal will pass through a light bulb. When it does, it experiences a drop in electric potential or pressure. Thus different colors are used to color wires located on the opposite sides of any bulb through which charge flows. Wires
further from the positive terminal should be colored with colors that are lower in electric potential.
If a wire
branches off another wire (like it does between the top two bulbs in the diagram), then continue the color into that branched wire until you reach a bulb. Do this because a branch does not change the electric potential (or pressure). Only a bulb is capable of changing the electric pressure.
You probably noticed there is another branching location near the bottom of the circuit diagram. It is important to remember that the electric pressure can only be changed by a bulb. Any wire leading into or out of a branched location will share the same electric potential and thus be colored the same color.