Why do some r10k configuration files start their YAML lines with :?

It is how foreman do there variable

:cachedir: '/var/cache/r10k'

will be read as

{
  ":cachedir": "/var/cache/r10k"
}

You can test your syntax here : http://yaml-online-parser.appspot.com/

foreman var : http://www.theforeman.org/manuals/1.5/index.html#3.5.2ConfigurationOptions