"Kiss Goodnight" is a phrase used to describe "going out on a high note". As Jimmybop so correctly noted, the castle starts changing color at dusk. That IS NOT the "Kiss Goodnight."
The kiss is usually an announcement or a closing presentation. At MK, Mickey now comes out to the train station to thank our guests as they leave. Technically, that is the official "Kiss Goodnight," although if the park closes with fireworks, that can be considered a KGN as well. The castle announcement is not done every night, but that is also a KGN.
At Epcot, IllumiNations is considered the Kiss Goodnight, but there is also sometimes an announcement in Future World as the masses are filing out. That too is a KGN.
At Studios, the KGN is an announcement that features the song "A Star is Born" from Hercules.
But the castle simply changing colors is just that: the castle changing colors. Spaceship Earth changes colors starting at dusk, but that is not the KGN.