EM wire colour codes and their traps
Each maker has its own colour abbreviations, listed on the schematic. On Gottlieb, BL-WH means blue with a thin white tracer, while BL&WH means blue and white mottled about equally. Bally sometimes adds a number after the colour, counting earlier uses of that colour. Gottlieb red often fades to look white; colours change after passing through a load; connectors are rarely drawn; and the printed colour is occasionally wrong.
Systems
Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)