Purée the raspberries with the grapefruit juice in a food processor or liquidizer until smooth, then press through a sieve and discard the raspberry seeds.
Heat the butter in a large bowl set over a saucepan of simmering water until melted. Stir in the sugar and raspberry purée, then strain in the eggs and mix together. Cook for 40–50 minutes, stirring from time to time, until the sugar has dissolved and mixture has thickened.
Stir in a few drops of food colouring, if liked, then ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool. Store in the refrigerator for up to 2 weeks.
To serve, this curd can be teamed with shortbread biscuits, whipped cream and whole raspberries for a dainty dessert.