For a sweeter cocktail you can easily use fresh orange juice, but if a more dramatic cocktail is what you're seeking, try using blood oranges in place of the pink grapefruit. They are in season from November to April in the U.S.
1/2 cup freshly squeezed pink grapefruit juice
1/4 cup citrus flavored vodka
1 tablespoon grenadine
1) Combine grapefruit juice and vodka in a cocktail shaker filled with ice. Shake for 30 seconds or until well chilled. Pour into a chilled martini glass. Slowly pour grenadine against the inside edge of the glass allowing it to settle to the bottom.