Why aren't our bones grey when calcium is grey?
Our bones aren't made of pure calcium metal, they're made of compounds that contain calcium. Bones have a matrix made mostly of collagen, a protein. The matrix contains calcium carbonate and calcium phosphate, both of which are white compounds that give bones their white appearance. Bones also contain living cells that produce new bone tissue.
Compounds usually have different properties than the elements from which they form. For example rust, which is iron oxide, is a different color than the iron and oxygen from which it formed. Sodium chloride, which is table salt, is white while sodium metal is gray and chlorine gas is greenish-yellow.
The elements that make up the human body are mostly in the form of compounds or aqueous ions. Besides calcium the body contains carbon, hydrogen, nitrogen, oxygen, phophorous, sulfur, sodium, magnesium, potassium, chlorine, and many other elements that are part of compounds. Most of these are too chemically reactive to remain as free elements.