The food colouring S.Martino shall be dissolved in water and added to the preparation. Based on the dissolved quantity you can obtain defferent shades that go from lighter to darker.
The "minimum" concentration to have an almost imperceptible colour is about 0.005% (1g of food colouring in 20 litres of water), from there on the colour becomes more and more visible. Due to high solibility you can reach high levels of concentration, therefore we suggest you to not go over 1% (10g of food colouring per litre of water).
The bitter taste is due to the salt contained in the food colouring. We suggest you to dissolve it with water and sugare to counteract the bitterness.