It's a really far stretch to say the Portuguese or most of the six countries "colonized" Ghana. They really just stuck to their trade ports and conflict was with other Europeans, not locals. It was the British who fully colonized Ghana.
ANY line running north-south between the Earth's poles and along the Earth's surface is a meridian. The one that passes through Greenwich is the Prime Meridian.
it’s not really an exact science and both are used nearly interchangeably in english, but emigration has the emphasis on leaving a place (to go somewhere else), and immigration is more to do with entering the new place. however, du bois did still emigrate with ghana as the destination, so either is probably fine.