Place shredded red and green cabbage in a large bowl. Sprinkle evenly with salt and massage thoroughly for 3–5 minutes until softened. Let sit for 45–60 minutes, turning occasionally, until the cabbage releases a good amount of liquid and becomes flexible. Rinse lightly under cold water and squeeze out excess moisture. Set aside.

In a small saucepan, whisk together sweet rice flour and water. Cook over medium heat, stirring constantly, until it thickens into a thin paste (about 2–3 minutes). Remove from heat and let cool completely.

To make the kimchi paste, combine cooled rice paste, garlic, ginger, gochugaru, tamari, and sugar in a bowl. Stir until smooth and vibrant red.

Add cabbage, radish, and green onions to a large bowl. Add the kimchi paste. Using clean hands, massage and mix until all vegetables are evenly coated.

Eat immediately, or prepare the kimchi for fermentation. Pack tightly into a clean glass jar, pressing down firmly so the vegetables are submerged under their own liquid. Leave about 1 inch of headspace.

Let sit at room temperature for 2–5 days, depending on desired tang and room conditions. Open the jar once a day to release excess gas. Taste after 2 days. Once it reaches your preferred flavor, seal and transfer to the fridge to slow fermentation. Enjoy!