Only visible at most a few degrees above the horizon during dusk, Mercury's tiny 7" gibbous disc shows up here in this colour image. The colours you see are due to shimmering in the Earth's atmosphere, known as scintillation.
Image information: Approximately true-colour 0.12s combined image in B, V and R filters with 1/2 the mirror covered during bright twilight. Image taken: 21:40 17/05/07.