Line an 8x8-inch baking pan with parchment paper, allowing it to hang over the edges for easy removal. Set aside.

In a medium saucepan, combine sweetened condensed milk, white chocolate chips, and half of the semi-sweet chocolate chips. Heat over low heat until melted and smooth, about 5-7 minutes.

Remove from heat and stir in peppermint extract until well blended.

Divide melted fudge into two bowls; mix crushed peppermint candies into one bowl and the remaining semi-sweet chocolate chips into the other.

Pour peppermint mixture into the prepared pan, then pour chocolate mixture on top, allowing it to cascade.

Swirl the two mixtures together gently using a skewer or knife, adding red food coloring if desired.

Chill in the refrigerator for 2-3 hours until set.

Remove from the pan using parchment paper edges and cut into squares. Serve or package as desired.