If you really don't want to change color, you can dilute the vegetable juice when you add it to the noodles, and reduce the concentration so that you can achieve the effect of solid color, or add a small amount of red currant powder to increase the color when you add it to the noodles.

Red currant powder is a natural pigment made from fermented red currant fungus cultured in rice, which is harmless to the body and can be consumed by the general public. But that will mask the flavor of the vegetables and reduce the nutritional content of the vegetables.