Use genuine concrete pigments, very colorful, when concrete casting in concrete moulds . To all dark objects, I have used 15 grams per kilo of concrete. You can mix in less pigment, but not more. If you do, the concrete may fall apart. How many kilo concrete do you need for your concrete mould? If you want them as black as possible, multiply the con- crete weight with 15 grams, or, If you want just a shade of grey, use 1 or 2 grams per kilo.
When concrete casting mix according to instructions on the package. Maybe a little less water, because you ́ll add water and pigment later. Stir thoroughly in five minutes, let it prove 5 minutes and stir again. Weigh the concrete, and multi- ply. Turn off the scale, add some water in a paper cup and put it on the scale. Start the scale and it will show 0 grams. Multiplie the concrete weight with 15 grams or less. Add the pigment to the water until you reach the sum. Stir the pigment and make a solid liquid without chunks. Pour the liquid into the wet concrete and mix accurately for several minutes. If you do it sloppy, you will propabely get stains.
When you remove the mold two days later it will look rather dark. Objects from concrete casting will dry and get a lighter tone when the concrete mould is removed.