Mix the sugar, salt and water in a pan and bring it to a boil.
Give it about 4 - 5 minutes from the point when it starts boiling, then switch off.
Let this syrup cool down, then add your freshly squeezed lime juice and a few drops of color.
Mix together well, then pour it into a dish. Cover and freeze it for a little while until it starts to set.
Remove the dish and, using a fork, scrape everything until it looks like crumbs, then cover and put the dish back.
Again a couple hours or so later, remove the dish and scrape it up once more and cover and return it back into the freezer.
This process gives your granita that lovely slushy icy crumbly look and texture.
Whenever you're ready to have it, scrape it up quickly again using a fork, then serve and garnish with mint or lime slices or even lime rind.