The colour of granite is determined by the variety and ratio of minerals within its make-up.
This is because minerals absorb different amounts of light wavelengths, which in turn gives each mineral a different colour – so, depending on which minerals are predominant within the rock, the colour and hue of that particular granite will sway towards the colour make-up of those minerals.
The 20-60 percent quartz is typically milky white or colourless, and the other minerals that are present then add the colouring or hue to it – this is why granite colours and patterns are so unique.
We will soon go into the mineral breakdown that determines each granite colour, but to give you a quick example – if magma cools and solidifies with a higher percentage of potassium feldspar compared to other minerals that are present, then chances are it will form a pink granite.
Black with white speckles is the most widely used granite colour, and this would mean that the magma solidified with a high percentage of quartz and amphibole.
It's all very scientific and if this is something that interests you then you may want to look into the study of mineralogy.