Some details I recall from this movie, which reminds me a little of the Carpenter film The Thing but was made much earlier.
It takes place largely on a space ship, and was made in the 1950s or 60s, judging by the aesthetic. It also was in color.
The crew is trying to deal with aliens who take over your body and pose as humans
In one scene the crew member who is "infected" is revealed because his jacket falls open and you can see he is a partial skeleton underneath, I remember the Big Reveal is that he turns around and faces his crewmates
Towards the end of the movie a crew member, thinking he is the last one left maybe? Says something like "I can't let you invade our world!" and smacks some kind of power source with an iron bar, electrocuting himself in an attempt to destroy their ship
The "big twist" at the end is two remaining crew, an older man and a woman, calmly state that their ship is damaged and can't fly much longer; they put up on screen a picture of the planet they will land on and -- it's Earth! I think they remark on how it's primitive but they will be able to act with no problems.
I saw this when I was really young, and the memory of the skeleton-under-jacket has stayed with me. I think I saw it on the Creature Double Feature (which Boston-area readers of a certain age might know).
I know it isn't "The Thing From Another World" which was in black and white and has rather differing plot elements.