How exciting that you are growing cabbage for the first time. You chose your seed well. Open-pollinated means that the plants are not hybrids. Non-GMO means that scientists did not artificially introduce genetic material from another plant or animal into the plants. Many consumers today are concerned with GMO (genetically modified organism) plants because no one knows the long term effects of eating genetically modified plants.
You can grow both red and green cabbage in the same garden if you are not planning on saving any seed to plant next year. If you allowed both the red and the green plants to flower, bees and other pollinators would cross-pollinate the plants. The resulting seeds would be hybrids and not look or taste like the parents. If you want to save seed for future planting, you should only grow red OR green cabbage to keep the strain pure.