Before I travelled to Hong Kong, I looked at the MTR Map, which described all the lines by colour and by name, but not by number. Generally, the signs up around the MTR did the same.
However, a few times, and especially when changing trains, I spotted signs with name + colour + number. This would've been easier for me, as in a jetlagged state remembering "I want the line that starts with a T" turned out to not be nearly so unique as planned...
Are there actually numbers for lines, or do the numbers mean something else? I didn't get a chance to take a photo, but below is a cc-by photo from Flickr which shows the kind of sign I spotted: