There is really no measure of whether a country was overall Positively or Negatively affected by the colonial era. Rather, there were positive effects and negative effects in different areas and to different peoples. A Black officer in the King's African Rifles could certainly consider himself positively impacted by the Colonial Era, though the same might not be said for a tribesman who found himself driven off his land to make way for a plantation.

For example, One could argue that Colonialism positively affected the continent by bringing with it:

• Modern Technology and Agricultural practices
• A Common Law Legal System
• A Central, Stable authority
• Democracy (Though this obviously wasn't always true)

Likewise, one Could Just as easily note the negative impacts:

• Artificially created borders leading to sectarianism
• The Transatlantic Slave Trade
• Resources were extracted with the profits going near entirely toward enriching the Colonizing power

Historians tend to avoid passing moral assessments on a topic so broad as this, simply because it clouds and simplifies the facts - That is instead more the focus of Politicians or Philosophers. Instead, one should simply look at the era's individual effects for what they are.