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I lived in Hawaii when Apollo 15 returned from the moon. I saw the crew return by helicopter from the ship to Hickham A.F.B. and when the helicopter carrier, the USS Okinawa (LPH 3), returned to Pearl Harbor, my father (a USAF full-bird colonel) arranged for my family to get a tour of the ship from one of the frogmen who helped keep the command module afloat and pull out the astronauts. He gave me a small piece of the gold-colored mylar foil which he said he and other sailors stripped off the outside of the command module as souveniers (I saw the capsule on the hanger deck). The sailor, I saw, had retained about a piece of foil about 9 inches in diameter.

The capsule did look clean as I saw it on the hanger deck, not like this photograph of Apollo 11 as it was hoisted onboard the Hornet; rather it looked more like the cleaned-up version of Apollo 11 in the Air and Space Museum. I suspect that the mylar foil was on the underside of white paint that burned off during reentry, and that the sailors finished the job. But I do not know this.

Is this piece of mylar I have MLI? Is it possible to have that verified? If it is MLI, what material would have covered it so that it did not look gold on liftoff? Was it always that color, or did its color change? Is it worth anything today?

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The outside of the Command Module is covered by a heat shield. According to this report (page 5), the forward portion of the heat shield (i.e. the part covering the cone-shaped part of the CM) is a 0.5" thick layer of phenolic resin. It is covered by several outer layers: a pore seal, a moisture barrier (which is white) and the outer layer is a silvered Mylar reflective coating. This looks like aluminium foil initially, as you can see in this photo:
Apollo CM in flight

After reentry and landing the outer layers are partially burned. This is the Apollo 13 CM during recovery:
Apollo 13 CM post-flight

The charring could have caused the Mylar to yellow, or the Mylar was yellow to begin with and burning off the reflective coating revealed its original color.

After landing, the CM would have been cleaned up for display, the charred outer layers removed to reveal the phenolic resin beneath:
CM in Science Museum

In conclusion, the piece of Mylar you have is not MLI.
On liftoff, the CM was covered by the white Boost Protective Cover which was attached to the Lanch Escape System. Once the rocket was above the atmosphere, the BPC and LES were jettisoned.

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Actually, it's Kapton (polyimide), not Mylar (polyethylene terephthalate) or multi-layer insulation. Kapton and Mylar are both DuPont trademarks for polymer films, so perhaps it is easy to get them confused.

According to the Apollo Experience Report: Thermal Protection Subsystem,

The thermal control requirements for the spacecraft in outer space necessitates a relatively low thermal absorptance-to-emittance ratio of 0.4 for the surface of the CM. This low ratio is achieved with a pressure-sensitive Kapton polyimide tape that is coated with aluminum and oxidized silicon monoxide and that is applied over the entire external surface of the ablator.

pp. 5-6

I think what @Hobbes is talking about is a layer that is temporarily added during manufacturing:

After completion of these operations, the main ablator is checked for moisture content. A layer of thin, epoxy-based pore sealer and a moisture-protective plastic coating then are applied to the surface to ensure sealing of the porous ablator.

p. 8

However, this plastic coating is later removed and replaced with the Kapton:

Before the CM is shipped from the prime contractor site to the NASA John F. Kennedy Space Center (KSC), the plastic coating is stripped off and the thermal-control coating with an adhesive backing is attached to the CM.

p.9

The phenolic resin underneath the Kapton varies in thickness from 0.7 to 2.7 inches, in various places around the CM. The rest of Hobbes' answer is correct.

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The mylar is yellow on one side and silver on the other. Today I was cleaning out the effects of my father who for 30-some years was security for Cape Canaveral I came across an envelope for he had saved a small piece of it and a piece of the original velcro strips used on the bottom of the astronauts shoes so they could maneuver in the capsule the inside of the capsule being lined with the soft portion of Velcro

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