The few times that you do see exterior shots filmed for TV studios during this time period, the color is always washed out and less vibrant than the studio shots. Blues turn to grays while reds turn to pinks. Color was very tricky in the early days of color TV. There were controls on interior shots with lighting and whatnot, but outside, the film crew had to fend for themselves.
Since the Dark Shadows studio shots were so clear, I think one of the reasons color footage was not used after the transition was because the color footage they had wasn't, and it clashed. It was probably washed out. It wouldn't matter when seen in black and white, but in color, it might not have looked very good when compared to the studio shots. This could be a reason that stills were used after the transition, as they were definitely clear and, if they were not originally, could be easily color-corrected. Another thought is night-imagery. It would be nearly impossible to get a good color exterior shot at night without a lot of expensive lighting. Since so much of Dark Shadows takes place at night, this could be another reason they decided to go for stills. The blue tint of the Collinwood night photos could be easily applied and the picture would still be crystal clear. Just some thoughts.