[PmbPhy (OP)]

I decided to watch he movie The UFO Incident which is the story of Betty and Barney Hill. The show the usual grey skin on an alien which was one of the reasons that made me skeptical of such stories since animals with red blood, assuming they have red blood (red from iron which helps blood carry oxygen). However when I note that an Elephant has grey skin it makes me ask what makes it grey?

[RD]

... I note that an Elephant has grey skin it makes me ask what makes it grey? ...

Grey is the color of African megafauna : elephants , hippos, rhinoceros. It's the optimum albedo to regulate temperature. 

Can lead to confusion though ...

[Kryptid]

I decided to watch he movie The UFO Incident which is the story of Betty and Barney Hill. The show the usual grey skin on an alien which was one of the reasons that made me skeptical of such stories since animals with red blood, assuming they have red blood (red from iron which helps blood carry oxygen). However when I note that an Elephant has grey skin it makes me ask what makes it grey?

Aliens could have any color of blood. They would have gone down a completely different evolutionary path than us and would no doubt have different oxygen-carrying chemicals in their analog of blood from Earth life. As to the question, apparently gray hair is caused by small amounts of black eumelanin. Something similar may be at work in elephants.

[Colin2B]

Can lead to confusion though ...

But if they are happy ..............

[SquarishTriangle]

Colour (as in skin pigmentation) could be energetically expensive to produce and maintain. Some animals use colour for camouflage or to confuse predators; others use it to attract a mate. But if you're one of the largest animals on land, have no need for camouflage, live in a herd, and competition for females exists in the form of big, strong males fighting, it is probably not worthwhile producing massive amounts of skin pigmentation. More useful to have skin that is resistant against UV exposure and helps maintain an ideal body temperature.

I am always puzzled why 'aliens' in films look so humanoid. Lack of imagination? Or do we really think that a life form that evolved in a distant planet/galaxy under completely different conditions would just so happen to look just like us, but with bigger eyes and forehead?

[RD]

But if they are happy ..............

What will the offspring be ?, hell if I know     

[Colin2B]

.....animals with red blood, assuming they have red blood (red from iron which helps blood carry oxygen).

Blood which has given up its oxygen eg venous blood, goes dark blueish. People who are short on oxygen eg congestive heart failure, lung damage, can have a pale, greyish skin colour.
Perhaps these aliens come from a planet high in oxygen? In which case why are they not seen wearing breathing apparatus? 

[Kryptid]

I am always puzzled why 'aliens' in films look so humanoid. Lack of imagination? Or do we really think that a life form that evolved in a distant planet/galaxy under completely different conditions would just so happen to look just like us, but with bigger eyes and forehead?

I presume that it's because (1) it costs less to put a human in make-up than it does to build a complex puppet rig of a non-humanoid creature, (2) humanoid aliens would be easier for the audience to understand or relate to, (3) there may be some human-centrism at work in the belief that our basic body plan is the best when it comes to an intelligent, technological species, and (4) abduction and alien encounter reports usually do feature humanoid aliens. Exceptions do exist, such as robotic beings and blobs, but humanoids are the most common.

[SquarishTriangle]

I presume that it's because (1) it costs less to put a human in make-up than it does to build a complex puppet rig of a non-humanoid creature, (2) humanoid aliens would be easier for the audience to understand or relate to, (3) there may be some human-centrism at work in the belief that our basic body plan is the best when it comes to an intelligent, technological species, and (4) abduction and alien encounter reports usually do feature humanoid aliens. Exceptions do exist, such as robotic beings and blobs, but humanoids are the most common.

All good points. The features of our basic body plan that include: forward-facing eyes (binocular vision), a bipedal gait, four limbs, prehensile hands, laterally-positioned but immobile ears, a dorsoventrally-flattened torso and a largely flat face do seem to imply a specific set of evolutionary selection pressures. Presumably, such a creature would have to have climbed some sort of tall, hazardous structure at some point in its evolutionary history, and then proceeded to develop the ability to balance itself on two limbs. And this is why I can't enjoy films like a normal person...

[PmbPhy (OP)]

.....animals with red blood, assuming they have red blood (red from iron which helps blood carry oxygen).

Blood which has given up its oxygen eg venous blood, goes dark blueish. People who are short on oxygen eg congestive heart failure, lung damage, can have a pale, greyish skin colour.
Perhaps these aliens come from a planet high in oxygen? In which case why are they not seen wearing breathing apparatus? 

If it was high in oxygen then it'd still be red/pinkish. Its not as if people turn grey when they'd breathing pure oxygen.

My motivation for this question was not a good reason to post it. Too much unjustified speculation.