As many other games using DOSBox on GOG, the installer for Colonization received yesterday an update to support the cloud saves feature.

The changelog text:

Internal Update (21 March 2019)
* [WINDOWS] added cloud saves functionality

now the game doesn't run for me; i push play button and after 5s it returns into its green form instead of launching the game

*an uninstall - install solved my problem

Was this with Galaxy? (I'm not sure if the offline ones were even modified).

They said in the news thread that some installers has gotten corrupted/there was an error will producing them, so they would be re-checking them. In the meantime, a verify/repair in Galaxy does the trick.