If you let a color TV display something from an unplugged display port, it displays snow, right? Randomly varying black or white pixels on the whole screen.

My guess was that the television was decoding the noise from the unplugged wires as a video signal.

Assuming this, it makes sense that B&W TVs make B&W snow (they would decode the noise as being a B&W video signal). But then color TVs should make colored snow, right? Shouldn't they decode the noise as being a color video signal?

If the receiver does not detect the sub-carrier for the "colour burst" signal which is transmitted during the horizontal blanking period the receiver switches on the "colour-killer" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise.



Figure 1. The colorburst signal is transmitted on the "back porch" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately enough for the remainder of the cycle and decode the I and Q information. Source: Maxim tutorials.

There's plenty of information on the web.

Links:


Compatible Color: The Ultimate Three-For-One Special by Technology Connections is highly recommended.
Signal Processing Stack Exchange: Why is TV static noise always black and white?.
Reenigne: Why is TV static in black and white?.

