In a 4-ounce canning jar, mix together the yellow mustard seeds, brown mustard seeds, vinegar, water, rosemary, grapefruit zest, and grapefruit juice. Cover and allow to sit in the refrigerator for 2 days. Pour the mixture into a blender or food processor with the sugar and salt and blend until it has smoothed out a bit but is still relatively coarse in texture. Keep refrigerated, use within 2 months.