"Never The Same Color" is an obsolete adage, originating from the early problems with NTSC analogue/broadcast chroma decoders.
It is certainly irrelevant to DVDs; you might say a DVD can be NTSC or PAL but all that really means is that the framerate and vertical resolution are compatible with those TV standards. The video is of course stored in digital YUV format.
Sorry I can't explain your general observations about PAL colours. Could you give some examples?