Combine sugar and 1/3 cup water in a small saucepan. Cook over high heat until simmering; stir until sugar is completely dissolved, about 2 minutes. Cool.
Combine cooled sugar syrup (should be 1/3 cup), lime juice, lemon juice and jalapeño in a blender. Blend on highest speed until pureed, about 1 minute. Pour into a pitcher.
Add tequila and triple sec to pitcher. Refrigerate until serving. To serve, run a lime wedge around rims of 6 glasses. Dip glass rims in white or green decorating sugar and fill glasses with ice. Pour margaritas over ice and serve.
