For C64, all sprites on screen share a global colour which may be the pink/skin tone colour (although, it could be the black).
However, it may also be down to the fact C64 is limited to 16 colours, and there would be a clash with the character and his blue background lab. Notice that none of the characters have an outline to distinguish from colour clashes.
The original DOS port retained the same 16 colour limitations of the C64 game. Some of the early 3rd party SCUMM engine resource tools and documentation refer to the first port using the same C64 compression scheme.
In later ports, the game went from CGA to EGA. I believe EGA is still limited to 16 colours but each room can choose from a greater palette of colours (i.e. could have 2 rooms with different palettes, but still limited to 16 colours per room). I'm not sure if each sprite can also choose its own palette too. It may be possible looking at your screenshots.
I suppose an art decision was made to make the residents more like b-movie characters in later ports. I can't see anything online to state why blue was chosen, but it does fit in with the B-Movie genre.
As for going back to a natural skin tone in the Day of the Tentacle release - this is either because the game included the original DOS port (which may be true as from memory you only had the original 1 save slot) or the developers wanted to use the same skin tones used in the Day of the Tentacle.