Step 1: Preheat your oven to 340°F (170°C) and line a 10×15 inch jelly roll pan with parchment paper, ensuring the edges are slightly raised.
Step 2: In a medium bowl, whisk the egg yolks with 30g of granulated sugar until pale. Add the vegetable oil and honeydew juice, whisking until well combined. If using food coloring, add it now to achieve a soft melon green.
Step 3: Sift the cake flour into the yolk mixture and whisk gently until no lumps remain. Set this mixture aside.
Step 4: In a separate clean bowl, beat the egg whites and cream of tartar until foamy. Gradually add the remaining 40g of sugar while continuing to beat until stiff, glossy peaks form.
Step 5: Gently fold one-third of the meringue into the yolk batter to lighten it. Then, fold in the remaining meringue in two batches using a spatula, being careful not to deflate the air.
Step 6: Pour the batter into the prepared pan and smooth the top. Tap the pan on the counter once to release large air bubbles. Bake for 12-15 minutes until the sponge springs back when lightly touched.
Step 7: Immediately remove the cake from the pan onto a clean kitchen towel dusted with powdered sugar. Roll the warm cake up with the towel and let it cool completely on a wire rack.
Step 8: While the cake cools, whip the heavy cream and powdered sugar until stiff peaks form. Gently fold in the diced honeydew pieces.
Step 9: Unroll the cooled cake, spread the cream and melon mixture evenly (leaving a small border), and roll it back up tightly. Refrigerate for at least 2 hours before slicing to set the shape.