These seven-segment displays are 6 inches tall and come in five different colors. The digits are controlled with a serial shift register interface, so multiple segments can be daisy-chained. Any number of segments can be controlled with three pins from an Arduino, Photon, or other controller.

For more detail, check out the project on the Reclaimer Labs website.
http://www.reclaimerlabs.com/seven-segment-display/
http://www.reclaimerlabs.com/blog/2015/3/3/prototype-clock
http://www.reclaimerlabs.com/blog/2015/5/5/using-an-arduino

All design files, including example firmware, are available on github.
https://github.com/AscendedDaniel/clock