You would not add any color to black to make grey icing, and once enough black coloring is added to reach a truly black icing, the only way to lighten it to grey would be to mix a tiny bit of the black icing into a great deal of white icing. The proper way to make grey icing is to simply add a very few drops of black to white icing to obtain the desired shade of grey.
whell since Blue is a primary colour i guess you can add black? perhaps.
Green
Red mixed with a little water will make a pink color. If you are making icing, add a drop of two of red food coloring to white icing in order to turn the icing pink.
No colors make black
Royal Icing is not a brand of icing to be purchased. It is made by using a recipe. It dries to a hard white, shiny gloss. Adding food coloring will make different colors.
Orange, purple and green are the secondary colors that can be made out of primary colors. You cannot make black out of primary colors.
black
Try black and blue. May work, normally just buy the colour I need.
Black....
When you mix red and blue, you get purple; for a lighter shade, you just need white, which the icing itself would probably supply. you start with white icing....... then add purple icing a little at a time...... to make purple, you mix red and blue. more red than blue though. If you use too much blue, it turns black......... now, the lilac icing will turn darker the longer it sits so you want to make it, ice the cake, then serve it right away....... It will turn the prettiest shade of dark purple the longer it sits.....
NO!!!!! All the primary colors mixed together make black and white. Black and white are in no way primary colors.