To note, the blue colors adds no nutritional benefit or added flavor - it's just for aesthetic purposes! First, drain and rinse your kelp noodles well. Use your hands to separate the noodles.
Add noodles to a large bowl of warm water. Add the tbsp of baking soda and immediately after add in the juice of a lemon or lime. Use your hands to massage the mixture then let sit for at least 10 minutes.
Add water to a pot and add in your red cabbage and the tsp of baking soda. Temperature should be medium-high. Let cook for about 8-10 minutes or so until the water has turned blue.
Turn off heat and remove the cabbage from the water. Add in your noodles and let sit until noodles have stained blue, about 20 minutes.
Drain your noodles and rinse. Be sure not to rinse too vigorously, the color holds on pretty well but can fade if washed too hard or hasn't soaked long enough.