Line an 8x8-inch tin with baking paper.
Break the white chocolate into chunks and place it into a saucepan with the condensed milk.
Place the saucepan over low heat on the stove and stir gently until the chocolate has melted. It should look thick and glossy.
Divide the fudge mixture equally between 4 bowls. Colour each one with food colouring- one black, one orange, one green and one purple. Add a few drops of the oil to each bowl at the same time to prevent the chocolate from seizing.
Spread the fudge into your lined tin, one colour at a time. If the fudge starts to thicken as you're working, you can pop it in the microwave for 10-20 seconds to make it smooth again.
Top with Halloween sprinkles then refrigerate for 4 hours until firm.
Remove the fudge from the tin, cut it into squares and enjoy!