The first thing you want to do is decide how many colours you want to use. Set the food colouring on the counter. Once you decide how many colours you want to use, you can set aside the same amount of small dishes/jars for mixing each individual colour. Set those aside.
Line a small pan with parchment paper, then spray with non-stick spray. Set aside.
In a large microwave safe bowl or measuring cup combine the white chocolate and sweetened condensed milk. Pop the bowl in the microwave for 1 minute, remove and stir, then pop back in for another 1 to 2 minutes, checking it every 30 seconds. Everyone's microwave is different so be sure to check every 30 seconds. It's done when the white chocolate melts smoothly into the sweetened condensed milk. It should be slightly thick, not runny. Do not over cook it.
Remove from the microwave and pour a little in each jar you set aside earlier.
Now add 1 to 2 drops of one colour in each jar. Use a popsicle stick to mix. Add the confectioners sugar equally into each jar, mix again.
Now you want to randomly drop each colour of fudge mixture into the pan. There's no rhyme or reason to it. Just drop colours here and there and keep layering them. Once they are layered you can take a toothpick or blunt edge of a knife and drag it through to make swirls or designs.
Place the fudge in the fridge for about 4 hours minimum, or overnight.
Remove from fridge and slice the fudge into small squares.