Can anyone tell me please how to colour plastic icing and buttercream icing cobalt blue, what colours and quantities will I need? Thanks

Not sure what you mean plastic icing?

Sorry, there's no "formula" for coloring fondant (plastic icing) or buttercream. It all depends on how your brand or recipe takes color.

It's difficult to get a very dark color without changing the consistency or, sometimes, flavor of icing when having to add a lot of color to it.

To make a very dark blue, I start with navy or royal blue Americolor gel and then add small amounts of black and sometimes purple to get it to the shade I need. I have also airbrushed the final coat of fondant in order to get exactly the dark blue that I needed.

HTH
Rae

Ahhh...never heard that term for fondant...but now I know!