How do I stop royal icing going hard once ive opened it?

[georgepig]

Im using shop bought royal icing but its white and i need it to be orange! Ive got some red and yellow food colouring (i presume thats right) and would like to 'colour' it today but dont need it until sunday, how do i stop it going hard?

Is it something to do with wrapping it in cling film but then where do i store it?

Thanks in advance

If you wrap it in clingfilm it will go hard any way (I know from DS having to ice and decorate a cake in school and only needed half the packet).
You could add some glycerine along with the food-colouring as you would for fondant icing.