Preheat the oven to 350°F/180°C and position a rack in the middle of the oven. Grease an 8x4 inch loaf pan with a little butter and line it with parchment paper leaving about 1 to 2 inches of overhang on both sides.
In a medium mixing bowl, sift 150 grams of all-purpose flour, 1 teaspoon baking powder, and 1/4 teaspoon salt. Whisk until well combined.
In a large mixing bowl, add 150 grams of unsalted, softened butter and 150 grams of castor sugar.
Using a handheld mixer, cream butter, and sugar on medium-high speed until light and fluffy, about 3 to 5 minutes.
Scrape down the sides and bottom of the bowl with a spatula once or twice.
Add eggs, one at a time, beating for about 1 minute on medium speed after each addition.
Add the last egg along with 1 teaspoon vanilla extract, beating for 1 minute on medium speed until well combined.
Scrape down the sides and bottom of the bowl with a spatula.
Add dry ingredients to the butter mixture in two parts.
Fold dry ingredients with the butter mixture gently after each addition, just until combined and no more flour streaks are visible.
Measure out 2/3 cup of the batter and transfer it into 2 small bowls. (1/3 cup batter per bowl)
Add a drop of red and green gel color to each bowl. Mix gently.
Spoon 1/3 portion of the white batter into the prepared pan, and smooth out the surface evenly using a spatula.
Next, spoon the pink-colored batter and smooth out the surface with a spatula.
Repeat with 1/3 portion of the white batter.
Repeat step 15 but now with the green-colored batter.
Add the remaining 1/3 of the white batter and smooth out the surface evenly.
Tap the loaf pan a couple of times on the countertop to eliminate any air bubbles.
Bake in a preheated oven at 350°F/180°C for 45 to 50 minutes or until a toothpick inserted in the center of the loaf comes out clean.
When the cake is baked, transfer the loaf pan to a wire cooling rack. Let the cake cool in the loaf pan for 15 minutes. Lift the cake out of the pan and allow it to cool completely on a wire rack. Once cooled, slice and enjoy with a cup of tea or coffee!