system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What allows for caesium, gold, osmium, and copper to have different colors while all other metals are gray or silver?
gpt
Conductive metals have an 'electron sea' of free electrons, which are constantly oscillating back and forth between moving toward and away from the positively charged metal ions; they are moving between energy levels. 

When light strikes the electrons, the electrons are excited to a higher energy level, producing some sort of light relative to the rate of oscillation. 

Most metals emit light in the UV spectrum, which we can't see and which appears silver/grey. Gold, however, has electrons that jump between the 5d and 6s orbitals. This causes more blue light to be absorbed than other metals, causing the gold color. 

The other elements mentioned also have unusual color properties due to particular numbers of electrons in particular orbitals.