Preheat oven to 350F
Beat together butter and sugar until fluffy.
Add eggs one at a time, beating well after each.
Add salt and baking powder and beat until combined.
Add the flour and the milk in 3 batches. Alternate 1 cup flour, 1/3 cup milk, 3 times.
Add The lemon zest and mix until all combined.
Split the batter into 3 or 4 equal parts, depending how many colors you plan to use. Mix a different color into each portion until you get the color you want.
Grease the bundt pan. Add about half of each color of cake batter to make the first layer of batter. Use a spatula or some other tool to swirl the colors around a bit.
Add the rest of the batter(s) to make a second layer. Swill this one as well. Tap the pan a few times to even it out and remove any bubbles.
Bake the cake for 50-60 minutes (until a toothpick comes out cleanly).
While the cake bakes, make the simple syrup. Stir together the sugar and lemon juice in a small saucepan over low heat, until the syrup has dissolved.
Remove the cake when done and let it rest/cool for 5 minutes. After 5 minutes brush the lemon simple syrup all over the cake. Use all of it, this gives the cake most of it's flavor.
Let the cake cool completely.