Works of art (paintings, statues etc) are copyright/trademark-protected by the original artist.
Any content featuring underlying artwork subject to copyright cannot be categorised as royalty-free material. If it is pictured in a larger context, it can be used as editorial content.
Please note that some art is illegal to shoot, e.g. the Sistine Chapel at the Vatican, and as such is not allowed on Colourbox. You should always do your research to ensure you are adhering to the differing policies.