THE JOYS OF SERIES. Years ago, strings of Christmas tree lights came wired in series, with house current powering a string of usually twenty low-voltage bulbs, each of which received 1/20 of the voltage and thereby burned cool enough so as to not set the tree on fire. Although the lights were a tremendous improvement over candles, when one bulb failed, that would disrupt current through the entire string and one faced the ritual of replacing bulbs one-by-one until the string again worked. If more than one bulb was bad, the frustration could be great. (The simplest fix would be to have a battery handy and some short pieces of wire to test each bulb. Those that lit could be returned to the string.)
After the Second World War, scaled-down, cooler-burning bulbs that could be wired in parallel became available. These strings pose a bit of a shock hazard as the full voltage is delivered to each socket, but when one bulb failed, it could easily be identified and replaced. At your store, these are the No. 7 colored bulbs.
The miniature lights that became available in the early 1970s were again wired in series, but with a circuit in each socket to shunt power around a bulb that failed, which would enable the owner to identify and replace a failed bulb. (I suspect that if too many bulbs failed at once, the shunts would not be able to protect the remaining bulbs from being fried by the higher voltage.)
To make maintenance easier, there is a gadget that enables the owner to work around a fried shunt. Voluntary Xchange marvels over this gadget.
There are other options. The small tree at the center of my train display has a string of 20 light emitting diodes powered by four batteries. There appear to be two circuits each of 10 diodes, wired in parallel. Service life of the diodes, 15,000 hours.