Explain why some types of magma form igneous rocks that are dark colored and dense.

1.the proportion of various minerals within the hot lava 
2. whether the magma cools under the earths surface i.e intrusive or on the suface i.e extrusive or even if they start to cool in mid air as the lava 
3.the speed at which the magma cools 

as far as i know the quicker the lava cools and the more air that combines with the magma the lighter the rock will be 
and the color i think is relevent to the mineral content of the magma 

I'm pretty sure it's c. extremely gradual but not 100% sure though.