Pour the mixture into a 3 quart heavy-bottomed saucepan and stir in the hibiscus petals. Cook over low heat, stirring constantly, until the mixture is thickened, about 10 minutes. The curd will thicken at about 170º F, or just below simmer. Remove from the heat and pour into a heat-proof bowl, scraping out any curd or hibiscus stuck to the bottom. Allow the hot curd to steep with the hibiscus petals for about 10 minutes, then pour it through the fine-mesh sieve into another bowl or an airtight container, pressing down on the solids to push the curd through. Make sure to scrape the curd from underside of the strainer. Allow to cool to room temperature, then cover the bowl with plastic wrap or seal the container, and refrigerate until chilled, at least an hour. Curd will get thicker as it cools.