I believe it is C-available by prescription but I am not too sure. Cocaine is used by physicians and for other medical purposes so it could be prescribed.
'Horse of another color' is the opposite expression from Shakespeare's 'a horse of that color' in the Twelfth Night Act 2 Scene 3 where Maria told Sir Toby Belch that her purpose was indeed a horse of that color which meant that Sir Toby Belch got the same idea as hers. So, 'horse of another color' is an idea different from the other.
