In a heavy bottomed saucepan, combine the sugar, corn syrup, and water over medium high heat. Bring to a boil, stirring to dissolve sugar.
Once the mixture starts to boil, stop stirring and insert a candy thermometer. Reduce heat to medium and continue cooking, without stirring, until the thermometer registers 260°F ("hard-ball" stage), about 10-15 minutes.
Meanwhile, in the mixing bowl of a stand mixer fitted with a whisk attachment, beat egg whites on medium high speed until stiff peaks form. The goal is to reach stiff peaks as the sugar reaches hard ball stage. I usually start beating the egg whites when the sugar reaches 230°F.
When sugar mixture is ready, remove saucepan from heat and remove thermometer. Gradually pour hot mixture in a thin stream over egg whites while continuing to beat the whites on high speed.
Add vanilla and food coloring (if using). Continue beating on high speed until candy starts to lose its gloss, 3-4 minutes. When beaters are lifted, mixture should fall in a ribbon that mounds on itself.
Test mixture by dropping candy by a spoonful onto waxed paper. If it stays mounded, it's ready. If mixture flattens, beat ½ to 1 minute more. If mixture is too stiff to spoon or has started to look a little grainy, beat in a few drops of hot water until candy is a softer consistency.
Immediately stir in walnuts. Quickly drop onto waxed paper, candy will harden within just a few minutes.
Store in an airtight container at room temperature, candy will keep for at least a week.