The reason black, blue, and red icing stain your tongue comes down to the type and concentration of food dyes used:

1. High pigment load : To get deep, saturated colours like black, royal blue, and bright red, bakers need to use a lot more gel/paste food colouring compared to lighter shades. That means your icing is packed with concentrated dye.

2. Dye chemistry : Many dark colours blue and red bases. These particular dyes (like FD&C Blue No. 1, Red No. 40) are very intense and bind easily to porous surfaces like your tongue.

3. Porous tongue surface : Your tongue has tiny papillae (like little grooves and bumps). The dye molecules get trapped there and stick more than on smooth surfaces, which makes the color linger.

4. Moisture + saliva spread : Since icing is moist and sugary, it dissolves and spreads the dye around your whole mouth, enhancing the staining effect.

👉 That’s why pastel or lighter colours don’t usually stain — they simply don’t contain enough pigment. 

** also note that if you find Reds, blues and blacks stain your tongue.. more than likely it will also stain stools**