"Pixel bleed" sounds like it isn't really doing it justice. More than 4 colors on CGA are usually achieved by using the composite output, which is actually plain NTSC.

In NTSC, color is added to the monochrome signal by modulating a quadrature amplitude modulated signal onto a carrier whose frequency is actually within the monochrome image bandwidth itself. NTSC did this for compatibility with black and white TVs of the time. (If that sounds too complicated, it "just" means that changing brightness very fast creates color.)[1]

To get more than 4 colors in CGA, you "just" switch pixels fast enough that you are effectively modulating the color signal yourself! But the problem with CGA is that it doesn't have a graphics mode with high enough resolution to get all those colors. But text mode does, so there's tricks to chop the text lines into pieces which allow puzzling back together a high res image that modulates the color signal.

It's pretty cool.

[1] PAL is the same with a twist, SECAM is very different but "changing brightness very fast" is still accurate.