Rinse the rose petals in cold water, and pat dry. Place 2/3 cup of rose petals in a saucepan with the heavy cream, half-and-half cream, and 1/3 cup of sugar. Place over medium heat, and bring the mixture to a bare simmer, stirring constantly. Remove from heat, and let the cream and rose petals steep for about 30 minutes. If you prefer, strain out the used rose petals. Set the remaining 1/3 cup of uncooked rose petals aside for later.
Preheat oven to 350 degrees F (175 degrees C). Spread pistachio nuts out onto a baking sheet, and bake until lightly toasted and fragrant, about 5 minutes. Remove from oven and let cool.
Prepare an ice water bath for rapidly chilling the ice cream mix: set a large bowl in the sink, and fill with ice cubes and enough water to let the cubes begin to melt. Place a strainer over a 2-quart bowl and set it in the ice bath. Have the ice water bath and bowl ready while you make the custard.
Place the egg yolks and 1/3 cup of sugar in a mixing bowl and whisk the yolks and sugar until thick and pale yellow.
Place the saucepan with the rose-scented cream back over medium-low heat, and bring almost to a simmer, stirring constantly. Gradually pour about a cup of the hot rose cream into the egg yolks, whisking constantly. Repeat with the remaining hot cream. Return the mixture to the saucepan and cook, stirring constantly, until the mixture is smooth, thickened, and coats the back of a spoon, about 5 more minutes. When mixture is just thickened, remove it from the heat.
Pour the hot custard through the strainer into the chilled bowl in the ice bath.
Stir the ice cream mix for about 2 minutes to prevent lumping, and mix in the rose water. Start with 1 teaspoon of the flavoring, and add more to taste, up to 2 or more tablespoons. Stir in food coloring, if desired. Place a sheet of plastic wrap against the surface of the ice cream mix, squeezing out any air pockets. Chill the ice cream mix for at least 2 hours or overnight.
Place the chilled ice cream mix into an ice cream freezer, and freeze according to manufacturer's instructions. When the ice cream is soft-frozen, scoop into a lidded container, and stir in the remaining rose petals and the pistachios to combine. Freeze, covered, for several hours to firm up before serving.
Look for rose water at Middle Eastern or Indian grocery stores.