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)	Red
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