Space probe imagery - why not in the visible spectrum?

Factual Questions
1

There was a very profoundly beautiful photograph of the earth, taken from Mars orbit, shown here:

http://www.msss.com/mars_images/moc/2003/05/22/

As usual, though, it’s color enhanced. This got me to wondering: why are most (if not all?) images taken from space probe grayscale, infrared, or something other than a color photograph in our visual slice of the EM spectrum?

2

IIRC light in the visual range is absorbed by interstellar particles. Xrays and infrared are more penetrating. There’s something called the water hole, and this site also says:

There’s a nice graph at that link, and Google will turn up a lot more.