It takes A LOT of color to turn fondant and/or gumpaste black. I use a combination of powder food coloring and black gel color and it will turn out reasonably well. Otherwise, gel food color alone, changes the consistency of the gumpaste in the volume you would need to get a true black.
The brands I use are CK (Country Kitchen) for the powder colors and Americolor gel for the gel black color.
If you need a large quantity I would just buy it ready made, if you need gumpaste specifically you can buy ready made fondant and add some tylose to make it gumpaste.
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