So the proper solution would have been to ethnically separate everyone out? Well, it's a nice idea, but (as I keep pointing out) it's damned near impossible. How does one draw an effective map of Africa when one has limited geographical/ethnic knowledge. Even if one does, drawing a border involves a lot more than simply drawing a line; people will be left on either side who do not like the line either for irredentist or personal reasons.
The matter was so complicated that when such a practice was tried in Europe in 1919 all the powers could manage was a series of 'best guess' options, most of which were subsequently overturned by the course of history. Only in 1945 did Europe actually manage anything like a clean up procedure, and there it was only achieved through mass population movement; much of it involuntary and in cases genocidal.
I can understand that, but you previous post seemed to suggest that the colonizers were not responsible for their own mistakes (because while perfect borders may have been unachievable better ones certainly were possible), I appologize if I missunderstood.