In heavy-bottomed saucepan, combine 3 cups water, sugar, and cream of tartar. Bring to boil over medium-high heat, stirring until sugar dissolves completely.
Attach candy thermometer to pan. Continue boiling without stirring until mixture reaches 240°F (115°C) - soft ball stage. This takes 15-20 minutes.
Reduce heat to medium-low. Slowly whisk in the cornstarch slurry in a thin, steady stream while whisking constantly to prevent lumps.
Continue cooking, stirring constantly with wooden spoon, until mixture thickens significantly and turns pale golden - about 20-25 minutes. The mixture should coat the spoon and leave a clear trail when stirred.
Remove from heat. Stir in rose water (or vanilla), lemon juice, and natural food coloring if using. Mix until evenly distributed.
Pour immediately into prepared pan, smoothing top with offset spatula. Let cool completely at room temperature for 4 hours or overnight until firm and set.