Which preposition goes with Aufsatz? Is it über or zu?
Ein Aufsatz über die Kolonialisierung von Namibia?
Ein Aufsatz zu der Kolonialisierung von Namibia?
They seem to be similar but are there any subtle differences between the two.?
Both are grammatically correct, but there is a subtle difference:
Please note also that zu is commonly fused with the following article:
Ein Aufsatz über can be translated as "an essay over..." That is covering the topic in a "general" mannner.
Ein Aufsatz zu can be translated as "an essay regarding..." That is making a point (or two) about the topic.
Either can be used, but the correct usage depends on the context.