I read Piranesi by Susannah Clarke recently which inspired me to draw classical statues. A good starting point for drawing practice, I have been trawling the British Museum, The MET, and google for images of sculptures. I started with value studies and a grid and now I'm venturing off the grid and using a wider palette.
Along the way, I came across this fascinating exhibition
I had heard that greek sculptures were painted and not left in the natural stone colour - but not to the full extent that this exhibition displays. The website has plenty of video, comparison examples, and text for easy viewing.