Yes. 2 ways that I know of.
1) the simplest and the way I think is best, convert it after you render. Just like a digital photo.
2) dont use color when you work. Just use grey values from pure black to pure white. IN which case you will need to copy all of your objects and make black and white textures.
But you probably mean, is there a slider or button in TG to render grey scale. I have never seen it if there is.
