On starting grayscale:
Yes, in a way it IS faster to start with grayscale. However, to turn grayscale images into convincing colour requires a good understanding of the effects of various layer modes in your drawing programme of choice, and oftentimes if you are not careful with colours you might end up with an image that is too dark or too saturated.
Additionally, colours often change the focus/shape of a picture, especially if you put saturated+neutral and/or cool+warm combinations together. Again, you need to plan the colours correctly, or you'll risk changing the focal point by accident.
However, values ARE important, since they are what makes a two-dimensional image look three-dimensional; various artists often recommend using a Desaturate adjustment layer, or a Color mode layer filled with white, as a sort of temporary screen over the image to check a digital painting's value range as you paint.
Of course, this is mostly moot in digital painting where anything goes, especially in the more complex programs like the full editions of Photoshop or Painter, where you can adjust your colours and values on the go. It comes down to preference and habit, really

Marta Dahlig and Da LuVisi, for example, paint straight away in colour, while Dave Rapoza starts in grayscale before building colour on top of of the grayscale image.