At http://phys.org/print398674581.html … Rosetta, during the most active phase of Comet Churyumov-Gerasimenko, caught evidence of carbon dioxide ice followed by the appearance of two large puddles of water ice nearby. The carbon dioxide ice layer covered an area the size of a football pitch and the two patches of water ice were each the size of a large swimming pool.
Another observation by Rosetta was a change in colour, towards the blue spectrum. This occurred as the comet approached the sun on it journey around the inner solar system. It is thought this change of colour was due to the removal of dust from the surface (blown out). This theory is just that, a theory. What is clear is that when the comet then began to pull away from the Sun it became noticeably redder in colour.