Line an 8-inch square cake pan with plastic wrap, leaving a 2-inch overhang on 2 sides. Combine white chocolate, condensed milk, butter, salt, and vanilla seeds in a medium saucepan over medium heat. Cook, stirring, until smooth. Remove from heat, and stir in both types of coconut.
Divide mixture among 3 bowls. Stir milk chocolate into 1 batch and food coloring into another. Leave third plain.
Spread milk-chocolate mixture into bottom of pan. Gently dollop plain mixture over top, covering as much surface as possible. Gently spread, then repeat with the remaining mixture. Refrigerate until firm, about 2 hours.
Unmold coconut square using plastic overhang, and discard plastic. Trim about inch each from 2 parallel sides. Cut crosswise into 1/4-inch-wide slices. Cut each slice into 1 1/2-inch-long strips. Cover, and refrigerate for up to 1 week.