I extracted the juice from fresh raspberries for colouring the icing pink, and from fresh blueberries and blackberries for colouring the icing purple. I did this by cooking the berries in a bit of water using low heat without boiling. Once the berries were soft and mushy, I removed from heat and mashed the mixture through a fine sieve to obtain the juice.
You may freeze it in ice cube tray for future use. The black/blueberry juice is a redder shade of purple so you may want to include some blue pea flowers if you can get hold of some. If not, you can leave it as it is or add a teeny dip of toothpick in blue liquid food colouring.
The iced gem recipe is the same as my original one but I will include it here for your convenience.
Plain biscuit recipe (makes about 65 biscuits)
70g plain flour
1 tsp baking powder
10g icing sugar
A pinch of salt
24g vegetable shortening
27g milk plus more as necessary
1/4 tsp vanilla extract
Berry royal icing*
140g icing sugar plus more as necessary (divide into 2)
10g meringue powder (divide into 2)
1 tbs raspberry juice plus extra as necessary
1 tbs blackberry and blueberry juice plus extra as necessary
A tiny toothpick dip of blue liquid food colouring (optional)
* If you are unable to get hold of meringue powder, you may prepare royal icing using egg whites. If you are concerned about using raw eggs, you may dry the iced biscuits in the oven at 90-100°C for half an hour to "cook" the egg whites as well as dry the icing.
1. Preheat the oven to 170°C using the fan mode. Line baking tray with baking sheet.
2. Sift together flour, baking powder, icing sugar and salt. Rub in the shortening with your fingertips until the mixture resembles fine breadcrumbs. Add vanilla extract to the milk, then gradually add milk to the flour mixture. Use a spatula to stir until a dough forms.
3. Knead the dough in the bowl until smooth. Roll it out on a non-stick mat or lightly floured work surface until about 3mm thick. Use a round cutter (about 2 cm diameter) to cut out the biscuits.
4. Place the biscuits on baking tray, flatten the tops with your fingertip and bake for 12-15 minutes or until golden brown.
5. Let the biscuits cool on tray for 5 minutes before transferring to cooling rack to cool completely. Store in airtight container or proceed with icing.
6. Sift together icing sugar (70g) and meringue powder (5g). Use an electric mixer to mix well on low speed. Gradually add raspberry juice and increase the speed to medium-high, beat for 5-6 minutes or until stiff peaks form. Transfer to piping bag fitted with a star stip. Pipe icing onto the biscuit base. Repeat for blue/blackberry flavour. If the icing is too runny at any point, just add more icing sugar.
My friend and her friend prefer this crunchy biscuit base to the softer, store-bought version. The flavours are pretty mild so if you prefer something stronger, you may have to add some artificial berry flavoured essence into the royal icing.
Update: check out my second attempt for black/blueberry iced gems with blue pea flower added to enhance the colour. The purple is deeper without having to add artificial colouring :).