Monochrome Pokemon began as a rumour. Here is the Marriland forum discussing them, and they're referred to as a 'texture error', which is either a small glitch or deliberately made by hackers.
This Game FAQs thread also discusses it, and monochrome Pokemon seem to appear in the wild/ on the Pokedex/ on screen, but then switch back to their original colour.
So to sum up, they are not another version of shinies, i.e. a super rare Pokemon, and they are not obtainable (or at least won't remain monochrome). If it happens naturally in your game, it should just be a colouring error and do no harm to the game itself.