Older Bell System wiring inside customer premises used a cable type with four conductors of solid copper wire with the insulation colors red, green, yellow, and black (legacy) or a two-color code of blue, orange, and white.OverviewThe 25-pair color code, originally known as even-count color code, is a used to identify individual conductors in for. With the development of new generations of telecommunication cables with polyethylene-insulated conductors (PIC) by for the in the 1950s, new methods were developed to mark each.