Answer:
In the ordinary sense of the word, the planet Mercury is not "luminous", that is, it does not radiate visible light like the Sun does and we ordinarily see sunlight reflected from it's surface.
However, like all bodies above absolute zero in temperature, Mercury radiates "black body radiation". Since Mercury is quite hot as planets go, it radiates more such radiation than most planets. This radiation is in the infrared spectrum though, so it is not "visible" to humans without artificial aids.
See the link.
When it comes to Hg, the element Mercury, the answer is pretty much the same except that Mercury vapor (in streetlights for example) IS luminous.