https://phys.org/news/2018-06-excessive-nasa-webb-telescope.html

The baffles essentially act as mirrors to reflect the heat (infrared radiation) outward in a specific direction. If that sounds familiar, it is because Webb's mirrors will do very much the same thing—but instead of reflecting the infrared light into space, they will guide it with pinpoint accuracy to the telescope's science instruments.

"Gold has a very high reflectivity in the infrared spectrum range, so it is ideal for directing heat," explained Matthew Stephens, a mechanical systems engineer for Webb at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is the same reason all of the primary, secondary, and tertiary mirrors are gold-coated."