You have to look closer because the colors of the film contrast with the details. That's also why you have to look closer in a lot of production stills, because they are done in a very similar photography as in the film. If you notice, both of the stills that show those details 'clear as day' are in a green-yellow tint, whereas the production stills not showing them are in other colors, principally in red-bronze-brown. A lot of the details are hidden in every single Alien film because of the cinematography. The skull in Alien, the ridge design in Aliens, a lot of the airbrushing in Alien3, the twin tubes, scanalatures and subtle ribbing in Alien: Resurrection to name a few.
And double posting is not allowed, so I suggest to edit your earlier post.