The alien that burst forth from ultra-white John Hurt was a deep black, does it therefore follow that if an alien burst from big black Yaphet Kotto it would have been white like alabaster?
I think they could make an attractive set of pets, like a black and white Labrador pairing.
Your irrelevant race-related comments aside, the alien bursting out from Kane's chest was flesh-colored. The filmmakers chose that color carefully, considering:
This is a horror scene which does not take place in the dark. On the contrary: the table is white and brightly lit with white light. Kane's shirt is also white. If you want to emphasize blood in an environment, the best background color is... you guessed it, white.
So the alien could have been as jet black as the adult alien at this stage, but the chestburster is flesh colored (almost white), to make the bloody hole in Kane's stomach stand out and to make the blood on the puppet itself clearly visible.
I think it was a brilliant touch by the filmmakers to have the scene brightly lit and have white background for it - it packs way more punch this way than having it happen in the dark. Of course they needed to get the effect shots 100% right as there is no way to hide imperfections / mistakes under such powerful lighting. Still they got everything right to make this scene legendary and iconic.
I don't think it has anything to do with the skin color of the host.
A white alien would be an interesting sight, however jet black was chosen for the alien for it to have an opportunity to blend in with the Nostromo's mechanical environments. The most prominent example being when it hid inside the Narcissus and popped out suddenly. Both the shape of the head and the color was similar to the pipes next to it. These scenes would not work with a white alien.
The best concept would be to have the alien to be colored after the colors of the national flag of the crew member. Come on, the Chinese alien would be red!