مُشَعْشَعَةً كَأَنَّ الْحُصَّ فِيهَا إِذَا مَا الْمَاءُ خَالَطَهَا سَخِينَا
A wine mixed with water: when water blends with it, its appearance suggests the reddish tint associated with ḥuṣṣ.
Mushaʿshaʿah describes a mixed or diluted drink. The commentary explains ḥuṣṣ by reference to wars, a plant-based colouring substance. The simile evokes colour; it need not say that this substance was actually added.
Kaʾanna explicitly introduces comparison, while idhā mā locates the visible change at the moment when water is mixed in.