Electric potential (sometimes referred to as electric pressure) can be indicated on a circuit diagram by colors that respresent a relative electric potential. The following colors are used:
 
A bulb will be lit as long as charge flows through it. And charge flows through a bulb if the wires connecting to its opposite sides have a difference in electric potential.
 

Coloring Wires:
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 along the top of the diagram. If a wire branches off the original RED wire (like it does along the top wire), continue the color RED into that wire and out to the next light bulb.

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. 

When you come to a branched location (such as along the top and the bottom of this circuit), then continue the color into that branched wire until you reach a bulb. This is done because a branch does not change the electric potential (or pressure). Only a bulb is capable of changing the electric pressure. So any wire leading into or out of a branched location will share the same electric potential and thus be colored the same color.

Identifying Lit Bulbs
Charge must flow through a light bulb in order for it to light. Charge will flow through a light bulb as long as there is a difference in electric potential (or pressure) on opposite sides of the bulb. So once you have done your color-coding, you will notice that there are several bulbs that are connected by wires of a different color (i.e., a different electric potential). These are the bulbs that are lit. Tap on a bulb to toggle it between its Lit and Unlit state.