Why are thiocarbonyl compounds often deeply colored and highly reactive?

Thiocarbonyl compounds, which contain the thiocarbonyl group ($\text{C=S}$), are often deeply colored and highly reactive due to the nature of the double bond between carbon and sulfur.1

This reactivity and color arise because the $\pi$ bond involves orbitals of significantly different sizes: the 2$p$ orbital of carbon and the 3$p$ orbital of sulfur. Because these orbitals do not overlap well, the resulting bond is weaker and more reactive than the corresponding carbon-oxygen double bond, which utilizes similar 2$p$ orbitals for both atoms.12

The poor orbital overlap and the resulting electronic structure are responsible for the distinct physical and chemical properties of these compounds compared to their oxygen-containing counterparts.2