Some people start out in B&W to get values and such right, and some prefer to start right away with colours. It really depends on what you're after. Sometimes colours can be the essential thing in an image, and other times the values won't be right in B&W because things can read differently in B&W than colours, without any of them being wrong as long as they keep to B&W and colour respectively.
Sometimes getting the colours right first and working the shape from there is the right way to go. Other times values make up the whole composition and to get them right doing it first in B&W is the way to go.
Some people find that they can't get things right without using B&W first, others that colouring after a B&W is faster than doing it all in colour - And then some can't get the colours right after doing a B&W or they end up kind of bland, or the values end up changing anyway after they start adding colour.
This also, of course, depends a lot on your level and what you are used to working with, and what is your primary goal in an image other than the subject.
What I often do as I work digitally, no matter if I've done a B&W image first or jumped straight into colour, is to add a B&W adjustment layer on top of my image, that I turn on and off. That way, while I work in colour, I can with a simple click check if my values are still reading right when it comes to focal points, areas, etc. Often I will notice something I forgot about or that I've worked on too much and ended up going in the wrong direction.
Hope that helps.