Wash the guavas well. Heat water in a large pot, once it comes to a boil, add the guavas. Cook until the guavas are tender, drain the water in a bowl and reserve it for further use. Once the guavas are warm enough to handle, peel as much of the skin as you can & scoop out the seeds.
Boil the seeds with 1/2 cup of the reserved water.
Pass the guava's through a fine mesh sieve, collect the pulp in a bowl. You may also puree and then pass it through the sieve. Also, pass the seeds boiled in water through the fine mesh sieve and collect the extract in the bowl containing the guava pulp. Weigh out the total pulp. You will need exactly the same amount of sugar or slightly more if the guavas are not very sweet. I had a total yield of 300 grams (1 & 1/2 cup guava pulp), and so have used 300 grams sugar.
In a dry pot, add the guava pulp and the sugar. Stir on medium heat to prevent it from burning at the bottom. Use a long wooden handle preferably to make stirring easier.
After about 10 minutes, the mixture will start bubbling, add in the lemon juice and continue stirring. If it bubbles vigorously then reduce the heat to low and continue stirring.
After another 10-15 minutes of stirring the mixture will have reduced and will begin to thicken. Add butter and the food color, and keep stirring till the mixture leaves the edges of the pan or when you scrape the mixture at the bottom, it stays apart for few seconds.
To test: Keep a bowl of ice cold water ready, take a little mixture and transfer it to the water, try to form a ball. If you can, it is done.
Remove from the heat and immediately transfer to a well-greased plate or tin. Smooth it out on top. Leave it to cool and set at room temperature for 6-8 hours. Cut into desired pieces and store in an airtight container. I got a total of 20 pieces of perad with a 1/2 inch thickness. You can reduce the thickness to 1/4 inch and get double the yield.