Cut the guavas in half and scoop out the seeds. Cook the seeds with 1/4 cup water for a few minutes. Strain through a sieve. Reserve the liquid and discard the seeds.

Chop the guavas and cook in a pan with 1 cup water and the reserved liquid till soft and pulpy.

Cool for a while and process in a blender till smooth. Weigh the pulp.

Return the pulp to the pan with the sugar (which should be half the weight of the guava pulp) and cook, stirring continuously, till the mixture begins to leave the sides of the pan and is of soft-ball consistency. Add the lemon juice, butter, and red food colour, if using. Stir well; remove from heat.

Pour the mixture on to a greased board or plate and spread evenly with the back of a greased spoon. Cut into diamond shapes when cold.