Therin of Andor wrote:
It's possible the cel painters had not even seen TOS, let alone have access to stills of the show. Remember the Gold Key comic artists had also never seen the show.
Alberto Giolitti never screened the series, but he worked from a stack of publicity photos from seasons one and two, which is why some famous poses of the series leads and Enterprise
can be found among the line art.
Were Filmation's colourists even based in USA?
At the time of TAS' production, yes.