If you haven't already noticed a lot of my artwork consist of black and white values, this is how I prefer to work and create studies. Sometimes taking out unnecessary color help create a more clearer vision of what you are trying to depict or portray in your concept. Adding colour tends to cloud an idea in my own opinion so I like to add colour later.
Some may prefer the opposite and start with colour as its a way to quickly create mood atmosphere and so on. This is just my way which I have integrated into my own personal workflow.