Question

What percentage of Africa had been colonized by 1870?

Answer 1

Around 10-15%

Explanation:

As you know, it was in the 1870's when the Scramble for Africa began, therefore there'd be little colonization in the area as they would just be starting. Unfortunately, I could not find an excellent map from Africa in the 1870's, I did however find one in 1880:

Since you say only "colonized", as in, the areas owned by Europeans only (thus removing the tribes), I would say that 10%-15% would be fairly accurate (this is of course also counting the Ottomans as European, primarily due to their capital being Istanbul (or Constantinople)).