Tough call on the color options, I think I would go with the indented portions of the tank being purple, then grey on the outsides with mostly yellow on top and back. Go white or yellow on the exhaust stacks to help break up the grey. this would leave the doors grey so your cloud can be white and lighting strike yellow. Keeping the portions of yellow, purple, white and grey in the same portions that you have everything else I think would be the key. Great looking stuff btw.