I know that this is like teaching grandma to suck eggs, so I apologise in advance to you all, but the difference in colour might have been down to the fact that it was once gilt ... although judging by Wayward's comments, and he's had a really close look at it so i trust his judgement, this might be unlikely. I can't tell the exact size, but with the openings at the top it's almost like a Monteith style punch bowl, which would explain why the interior once may have been gilt ...
That's the best I can offer, I'm afraid.