QColorOutputPrivate::isColoringPossible checks whether we can use colored output (on non-Windows machines) using isatty:

return isatty(fileno(stderr));

This works fine when stderr is a regular terminal. However, when the output is passed through a pipe we lose the information about color support of the terminal that will display the output. 

This can happen, for example, when calling ninja all_qmllint. Ninja uses pipes to pass the output and isatty() return false.

This does not happen when running the individual qmllint commands manually and they have colored output.