Preheat the oven to 220C/200C(fan)/Gas 7. Grease two large baking trays with butter.
For the choux pastry, put the butter and 150ml/5½fl oz water in a small pan over a low heat. Bring slowly to the boil, tip in the flour, then remove from the heat. Stir vigorously.
When the mixture becomes a smooth paste, return the pan to the heat, stirring constantly. The mixture will dry out a little and form a soft ball that comes away from the sides of the pan.
Remove the pan from the heat again, leave the mixture to cool slightly, then gradually add the eggs, beating really well between each addition until the mixture is smooth and glossy. (You may not need all the eggs, the mixture should be a piping consistency.)
Sprinkle the baking tray with water (a water spray with a fine nozzle is good for this).
Spoon the éclair mixture into a large piping bag fitted with a 1cm/½in plain nozzle. Pipe the mixture onto the baking sheets into 7.5cm/3in lengths, leaving room between each éclair for them to spread a bit.
Bake for 10 minutes, then reduce the heat to 190C/170C(fan)/Gas 5 and bake for a further 20 minutes. Split each éclair in half lengthways and transfer to a wire rack to cool completely.
For the filling, whip the cream to soft peaks. Divide the cream into separate bowls and fold the lemon curd into one bowl of cream and the raspberry powder into the other.
Once the éclairs have cooled, spoon the lemon cream into a piping bag fitted with a 1cm/½in plain nozzle and pipe the lemon cream into the bottom half of six éclairs, then spoon the raspberry cream into the piping bag and pipe into the bottom half of the remaining éclairs.
For the icing, place half the icing sugar in a bowl and mix with enough lemon juice to form a very stiff icing. Colour with the yellow food colouring and spoon into a disposable piping bag.
Tip the remaining icing sugar into a bowl and add enough water to make a very stiff icing.
Colour with the pink food colouring and spoon into a disposable piping bag. Snip the ends off the piping bags and pipe on top of the raspberry éclairs. Spread the icing out using a palette knife dipped in boiling water, so the top is smooth.
Repeat with the yellow icing on the remaining six eclairs. Place on top of the lemon cream filled eclairs.
Drizzle the melted white chocolate over the raspberry éclairs in a zig zag pattern. Drizzle swirls of melted dark chocolate over the lemon éclairs. Serve.