Combine 1 cup grape juice and sugar in a small saucepan and sprinkle with gelatin. Let gelatin stand 1 minute to soften.
Cook over moderate heat, gently stirring occasionally, just until sugar and gelatin are dissolved. Remove from heat and stir in Elderflower Pressé, food coloring, and remaining cup grape juice. Pour into a 5-cup mold and chill, covered, until firm, at least 8 hours.
Dip mold in hot water a few seconds, then invert a plate over gelée and flip gelée onto plate.
Cooks' note: Gelée may be chilled (in mold) up to 2 days.