Valmiki Ramayana Yuddha Kanda Sarga 91 Shloka 46

निमित्तगाः having penetrated, रुक्मपुङ्खाः with red feathers, ते they, तस्य his, कायम् body, भित्वातु piercing, लोहितादिग्धाः red coloured, रक्ताः red, महोरगाः huge serpent, बभूवुः like

The arrows with red feathers of Vibheeshana having penetrated the body of Indrajith, piercing into the body they became red coloured seemed like a red serpent.