[Implemented] modification to Active Ports Screen

[Captncrunch]

So glad Captain_Jack pointed out the Active Ports Screen.

Always looking for improvements (to a very fine game), I can see a slight modification to Active Ports Screen that would make it more readable: The cost/stocks numbers for goods being bought could be coloured differently from those being sold, making it easier to read and compare ports at a quick glance (rather than having to keep switching between looking at the numbers and the productions list). I know this is a small change that might seem insignificant to some (most?) but I'm thinking that improving the readability could really help make for quick decisions and less headaches.