NASA's webpage: "The History of Spacesuits" explains that the suits for the Mercury program were an early design and not puncture resistant nor as heavy as the white suits (which contain aluminized mylar underneath). They were also not fire resistant.
"What Were Early Spacesuits Like?
NASA astronauts first flew into space during the Mercury program. NASA’s first spacesuits were made for Mercury. The Mercury suits were only worn inside the spacecraft.
NASA’s first spacewalks took place during its second space program, Gemini. The suits used for Gemini were better than the Mercury suits. But the Gemini suits were simpler than today’s suits.
Astronauts wore heavy white spacesuits when going on spacewalks outside the space shuttle. These same heavy white suits are sometimes used when astronauts go on spacewalks outside the space station.".
Due to a fire and resulting deaths the suits were improved.
Wikipedia - "Apollo Program":
"Astronauts Grissom, White, and Roger B. Chaffee were wearing A1C suits on January 27, 1967 in a preliminary countdown demonstration test for the planned February 21 Apollo 1 launch, when they were killed in a cabin fire, leading to NASA cancelling manned Block I flights and use of the A1C suit. Since the fire had burned through the suits, NASA added a fireproofing requirement to the new suit, which replaced the outer layer with beta cloth. The Block II suit was designated A7L and manufactured by ILC Dover. The new suit was first used on Apollo 1's replacement flight, Apollo 7 flown by Schirra, Eisele and Cunningham in October 1968.".
NASA has a couple of webpages explaining why the suits are white, but even webpages mentioning aluminized mylar don't say why is is better (or worse) than white.
- The reason that spacesuits are white is because white reflects heat in space the same as it does here on Earth. Temperatures in direct sunlight in space can be more than 275 degrees Fahrenheit.
Shuttle spacesuits are made by sewing and cementing various materials together, and then attaching metal parts that let the different components be joined together.
Shuttle spacesuit materials include ortho-fabric, aluminized mylar, neoprene-coated nylon, dacron, urethane-coated nylon, tricot, nylon/spandex, stainless steel, and high-strength composite materials.
Modern suits use multi layer insulation and thermal micrometeoroid garment for the outer layer:
A NASA Report on the testing of various materials titled: "Space environmental effects on spacecraft: LEO materials selection guide, part 2" on page 10-147 explains that FEP Teflon has a lower erosion from atomic oxygen than many other materials, and has this reflectivity chart on page 10-3:
White is almost as good as aluminized ("silver") color and can be applied to more durable materials, leading to an improved system overall.