The term "opaque" describes a color that cannot be seen through because it blocks light from penetrating the object or traveling through the object. Any color may be opaque and this term does not describe one specific color.
An opaque color helps to hide mistakes in art because once the opaque color is applied to a painting, anything underneath that color is no longer visible. Opaque treatments are also helpful for things like windows when one wants to create privacy. For example, an opaque finish works well on bathroom windows because it prevents those on the outside from seeing into the bathroom.