Most analog TV effects were done in a luminance/chrominance color space, like Y'CbCr (sometimes mistakenly called YUV). In these types of color spaces color is defined by its luminance (referred to by the variable Y) which is similar to the brightness of the color. (Brightness is a poorly defined word, unfortunately, so there's no solid scientific definition of it.) The chrominance is the hue and saturation of the color and is defined by 2 other color channels. They roughly represent the amount of red and blue in the signal. (They're the difference between the luminance and the blue channel, and the luminance and red channel respectively.)
When you decrease just the luminance value in a Y'CbCr color, the colors will become more saturated as they become darker. If you want to maintain the saturation as you decrease the luminance, you need to also move the Cb and Cr channels proportionally closer to 0. Similarly, if you change the contrast of just the luminance channel, the colors will become more saturated as you decrease contrast and less saturated as you increase contrast.
If you have access to a copy of Apple's Motion, they have a "Contrast" filter where you can choose to only affect the luminance channel. It shows the effect nicely.