Roberts Food Powders 

Oil based powder dyes are used to colour white chocolate. Simply add a few shakes of powder to your melted white chocolate and mix. Keep adding more powder until you get the desired colour. The powder has no taste.

When colouring chocolate BLACK, you need to work in very small batches (1-2 Tablespoons at a time) to achieve a really strong black colour.

Otherwise, you will end up with a more purple/mauve tone of colour.

You can also use these powders to colour icings including: Ready to Roll Fondant icing, butter frostings, modelling paste, flower paste.  
You can make a liquid paste but adding a drop or two of boiling water to some powder colour or mixing the powder dye straight in and kneading the colour through.