Colormeleons

The intended way of earning this achievement back when it was released was to use a light switch hidden behind the paintings left of the starting area. However, using the light switch in the most recent version does not grant the achievement.

In order to earn it with the abovementioned method download the September 2020 depot files by opening steam console and using the following command:
download_depot 1190990 1190991 1530439761091436968

Upon finishing the download you will receive a path to the downloaded depot. Open it and copy files from the depot (if asked choose to replace existing files) to the steam game folder.