Studio monitors (at least the "modern" version) are boxes with stickers that says "Studio Monitor" on them.
But assuming we're talking about the difference between the average nearfield speaker and a bookshelf stereo speaker of similar size - Studio monitors are narrow dispersion, limited range, short-throw boxes that are made to sound decent from a yard away (therefore, taking the room acoustics out of the equation to some point).
They're not "uncolored" - They're not "more accurate" - They're anything but "better sounding" (IMO) than a quality audiophile HiFi speaker.
Of course, there are really great sounding boxes out there - Most mid-sized Genelec, ADAM, etc. And of course, most of those are designed as mid-fields...
The whole concept of using nearfield speakers as mains is a pretty new concept - One that I've never been crazy about.