I always had this question but bored to ask what's the deal with old EGA cards that output rgb and comosite? it seems that composite has more colors, 16 to be exact, instead of the 4 rgb ones... see here for some examples:
I think you mean CGA. EGA has 16colors out of 64colors onscreen. CGA has 4 colors onscreen out of 8 (some sites say 16 but I don't remember having a palette that big with my old 8bit ISA composite output card). I also don't remember composite output with EGA cards, just CGA cards.
That's pretty cool though, getting 16 colors onscreen with the CGA card in 320x200 mode via color artifacting (or using 640x200 B&W mode to get a different set of 16 colors onscreen) - pic