Line an 8 by 8 inch glass baking dish with aluminum foil.
In a small bowl combine 1/2 cup white chocolate chips and 3 TBSPs plus 1 tsp sweetened condensed milk. Microwave for 30 seconds and stir until smooth. Stir in 4 drops of red food coloring and 4 drops of blue food coloring to make your purple layer. Spread it as evenly as possible. Set the fudge in the refrigerator to cool while working on the next layer.
Repeat the process adding instead 8 drops of blue food coloring. Continue the process until you have made and added all six colors. I used 16 drops for red. (Note: I did each layer one after another, but you might have to wait in between to not overheat your microwave.)
Once the fudge has completely cooled, remove it from the pan, peeling off the aluminum foil. Cut the fudge into squares and store outside of the refrigerator for up to two weeks in an airtight container.