A green paper released by the European Commission is a discussion document intended to stimulate debate and launch a process of consultation, at European level, on a particular topic. A green paper usually presents a range of ideas and is meant to invite interested individuals or organizations to contribute views and information. It may be followed by a white paper, an official set of proposals that is used as a vehicle for their development into law.
A reference to a green or white paper can be helpful to argue for the need of research in a particular field. But it is not a problem for if you cannot make a link from your proposal to a policy paper.
Below you find a link to a list containing all Green Papers published by the European Commission (EC) since 1993 - and some from before then. If the full electronic version of the text is unavailable, the link will take you to a summary in the Bulletin of the European Union.
Here are the links to find Green Papers and White papers.