Siebe is probably talking about my analogy of a picture drawn with dark colors to show how a pure citta is contaminated by asobhana cetasika in a post at: “Living Dhamma – Fundamentals“.
It is just an analogy. The Buddha used many such analogies to clarify things. Don’t take it literally.
Issa (jealousy) or dosa (hate) cetasika will of course darken (defile) a mind.