You can always make a colour image black and white later, but you can't turn a black and white image back into colour!
The other important reason to shoot colour is simply that if you shoot using the monochrome setting on your camera (throwing away the colour data), you'll never be able to get that colour data back, even if you decide you actually want a colour version of the photo later. You can always make a colour image black and white later, but you can't turn a black and white image back into colour!