What color mixed together will result in green?

In picassos masterpeice bulls head which bicycle part makes up the bulls horn

To make an etching the artist first coats the entire printing plate with an acid-resistant substance called a made from beeswax, asphalt, and other material. the artist then draws on the coated plate with an etching needle exposing the metal. next the plate is dipped in the plate is removed, the waxy coating removed, and the plate is cleaned, inked and printed.

While fragileworks of art might be best preserved in a locked box, museums routinely display such objects. why do museum professionals feel it is important to share these artifacts?