To retain the vibrant green color of the spinach sauce, blanching the palak leaves with a pinch of sugar is the secret. Blanching just brings out the color of spinach leaves. Once blanched, immediately drain the hot water and rinse the leaves thoroughly in ice cold water to stop the greens from cooking further. By doing this you will be able to retain the vibrant color of paneer palak. Also once you puree the blanched palak, use it immediately in the curry. If you keep the pureed spinach for long time as such, it will discolor.