For the citrus: Slice the Meyer lemons into 1/4-inch-thick wheels, then trim the wheels into even squares (about 1 1/2 inches in size). Repeat with the navel oranges, tangerines, Cara Cara oranges, grapefruits and blood oranges. Line the cut citrus pieces on a baking sheet in a single layer. Set aside.
For the glaze: Place the granulated sugar, brown sugar, butter and water into a small saucepan and bring to a simmer over medium heat; simmer until the sugars dissolve. Pour the mixture into a 9-by-13-inch baking dish and, using a pastry brush, brush over the entire surface.
Starting with blood oranges and beginning at the top right corner of the baking dish, tightly line the blood orange pieces side by side, with the corners at the top, bottom and sides so you have a row of diamonds, not squares. Repeat, fitting the second row tightly into the gaps from the first row. Repeat with remaining citrus, going from dark to light and creating 2 rows for each type, until the entire baking dish is filled. Cut the remaining citrus diamonds in half and place into the edges of the dish to fill in the remaining gaps around the border. Set aside.
For the buttermilk cake: Place the flour, baking powder, baking soda and salt in a mixing bowl and whisk together. In another mixing bowl, combine the butter and granulated sugar and beat together with an electric hand mixer until light and fluffy. Add the eggs one at a time and beat together. Add the dry mixture to the wet mixture and beat together. Add the buttermilk and beat together until a thick batter forms with no lumps. Stir the vanilla and citrus zest into the batter until just combined.
Pour the cake batter over the arranged citrus in the baking dish and spread around until you have an even layer. Bake the cake for 30 to 35 minutes or until a toothpick comes out clean when inserted into the center of the cake and the top is golden brown. Remove the cake from the oven and allow it to cool for at least 30 minutes.
Place a cutting board or large platter upside down over the top of the baking dish and quickly but carefully invert the cake. Carefully lift the baking dish from the cake to reveal the ombre top. Slice and serve.
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