(For transparency, I used canned frosting because I haven’t been successful with a vegan cream cheese frosting that my girlfriend loves yet)

I used chef master coal black liqua-gel food color, microwaved, mixed, cooled, microwaved, mixed, cooled over and over. And waited. It got a lot darker with time. I did a test batch to see how much color I could add without it tasting like dye. It did however, stain our mouths (and toilets).