Preheat oven to 350°F. Thoroughly grease a 10- to 12-cup Bundt pan, or spray with nonstick baking spray. (See Tip 4)
Beat together the butter and sugar until fluffy and lightened in color.
Add the eggs one at a time, beating well after each addition. Scrape the sides and bottom of the bowl once all the eggs have been added, and beat briefly to re-combine any residue.
Add salt and baking powder; beat to combine.
Add the flour to the batter in three parts alternately with the milk, starting and ending with the flour. Mix until everything is well combined. Stir in the lemon oil or lemon zest.
Divide the batter into thirds, roughly 16 ounces each. Color one purple, one green and one yellow with gel food coloring.
Spoon a layer of purple, green, yellow, purple, green, and yellow batter into bottom of bundt pan. Carefully swirl with butter knife. Tap pan several times on counter to evenly distribute. (See Tip 5)
Repeat the first layer, but beginning where the purple and green meet on the bottom layer. Swirl with a toothpick. Tap to evenly distribute. Repeat again for the third layer.
Bake at 350° F for 50 to 60 minutes, or until toothpick inserted into the center comes out clean.
Remove the cake from the oven, and carefully run a knife between cake and pan all around the edge. Place the pan upside down on a cooling rack set over a rimmed baking sheet. If the cake drops out of the pan onto the rack, remove the pan. If the cake doesn’t drop onto the rack, let it rest for 5 minutes, then carefully lift the pan off the cake. If the cake still feels like it's sticking, give it another 5 minutes upside down, then very gently shake the pan back and forth to loosen and remove it.