Can I mix colour and black & white pages in my document?

Yes. You may have pages that are colour images and/or text as well as pages that have grayscale images and only black text. Make sure that the grayscale images are placed in the document as grayscale and not RGB/CMYK that looks B&W. The print engine will see the colour information in the RGB/CMYK images and would consider those pages as colour pages and you would be billed as such.