Do I really need to buy green AND red lentils?

No. You could use one kind of lentils and adjust the recipe based on the cook time of the lentils you choose. Green lentils tend to take longer to cook, yet have a great peppery flavor and hold together better. Red lentils are more nutty, cook faster and can turn to mush if overcooked. Each recipe using these specific kinds of lentils was made by incredible recipe creators. I encourage you to get both and make the recipes just as intended if you’re up for it.