Painting freehand on a sheet of white chocolate is a way for the artistically inclined to create one-of-a-kind designs for special occasions. The artist needs to remember that white chocolate is non-absorbent, so it's like using watercolours on a glossy surface. Cocoa butter is used as a colour medium, because it is the same fat found in the white chocolate and will adhere well. Cocoa butter cannot be used with water-based colours, and the tinted "paint" will need to be kept warm as you work.
Break or chop the cocoa butter into small pieces and place them in a heatproof bowl. Set the bowl over a small pot of gently simmering water, making a double boiler. Melt the cocoa butter gently until it has liquefied.
Pour a small portion of the cocoa butter into a small, warmed bowl. Add a modest amount of gel colouring. You may need no more than the amount that will stick to the end of a toothpick depending how vivid a colour you want to create.
Repeat until you have as many colours as you need. They will solidify as they cool, so keep your colours warm by leaving them in a pan of warm water or sitting on an electric heating pad, when they are not in use. Alternately, you can use one colour at a time, but this is more difficult.
Paint on your white chocolate with fine brushes, using one for each colour. Do not clean the brushes with water as this will cause the cocoa butter to solidify. Your brushes must be completely clean and dry when you begin.
Tightly wrap your colours with plastic film wrap when you are finished and store them in a cool, dry place. When you wish to reuse them, simply melt them in a pan of warm water until they have liquefied once more.
Paint a small plaque of white chocolate to use as a decoration on a cake. It is much easier than painting the cake itself, and if you make mistake you need only melt enough chocolate to provide a new work surface. Cocoa butter colours can also be used to paint onto acetate sheets or ribbons. Pour melted white chocolate onto the sheet and once it has cooled the pattern will come away on the chocolate, leaving a glossy, professional-looking design. When you are painting on acetate, the colours will appear in reverse order when you remove the acetate. It's rather like painting on the inside of a window, you'll need to do the small details first and then progress to the background. An advantage of the acetate method is that you can print a copy of the design you want, put it underneath the acetate, and then trace directly over the design. For less demanding uses, like filling in the design in a candy mould, you can use coloured "melting wafers" or tint white chocolate to the desired colour with your gel colourings. Paint the details in the moulds one colour at a time, refrigerating for a few minutes after each colour. Fill with the white chocolate, harden, and unmould.