Ducky's tummy changes from yellow to almost white to light green throughout the franchise and her back is either (dark)green or a brownish color (LBT 6 for example).
Same problem with Littlefoot, Ali!, Cera and Petrie. Esp. Petrie has many different colours for his tummy... In the original it's purple, I think he was light red, light green, light blue and light yellow in different sequels...
The animation style indeed changed strikingly. While the original, having the least technical options, has a great and carefully done animation, not to mention the lovely drawn backgrounds, sequels 2-6 had a bad style imo. Compared to the original, it's far worse, the animation isn't great and the background isn't half as good.
7-10 were okay and 11 and especially 12 were quite well done and the backgrounds look far better.
Well... 13... as good as 7-10 I think.
The Tv-series is as bad (probably even worse) animated as (than) 2-6.