Reading some of the computer history books gives some insight into
that. It depended on the compnay, graphics chips back then were pretty
primitive either TTL logic or if there was sone hotshot designer in the
compnay maybe a chip. Though it took a hundred thousand (or more) to
design a custom chip.
Commodore had the in-house guys from MOS to do the work and developed
thier own chips, Atari had also lots of arcade machine experiece to get
their job done. Apple had color, but conparitively the Apple ][
actually had a pretty primitive color system (it didn't greatly improve
until the 80 column board)