Transformer Lead Color Code

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils or "windings". Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magnetic field in the core (or cores) of the transformer. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutual induction. - Wikipedia

The following table represents the most common color coding for transformer leads.

Power Transformers
Primary Leads Black
If it is tapped:  
                   Common Black
           Tap Black / Yellow
            End Black / Red
High Voltage Winding Red
(center tap) Red / Yellow
Rectifier Filament Winding Yellow
(center tap) Yellow / Blue
Filament Winding #1 Green
(center tap) Green / Yellow
Filament Winding #2 Brown
(center tap) Brown / Yellow
Filament Winding #3 Slate
(center tap) Slate / Yellow
IF Transformers
Plate Lead Blue
B+ Lead Red
Grid (or diode) Lead Green
A-V-C (or ground) Lead Black
Tuning Slug Color Codes
455 kHz    1st IF Yellow
 455 kHz    2nd IF White
455 kHz    3rd IF Black
     455 kHz    Oscillator Red
10 MHz    1st IF Green
10 MHz    2nd IF
10 MHz    3rd IF
Orange, Brown, or Black
Audio Transformers
Plate Lead, Primary Blue
B+ Lead, Primary
(w/or w/o center-tap)
Plate Lead
(center-tapped primary)
Brown (or Blue*)
Grid Lead, Secondary Green
Grid Lead
(center-tapped secondary)
Yellow (or Green*)
Grid Return, Secondary Black

* Push-pull only