To give your rhubarb plant a healthy start, avoid harvesting it the year you plant it. So you know for future harvests, there are many different rhubarb cultivars, and not all of them are red. Some selections have green, pink, or speckled petioles (stalks). Stalk color is not an indicator of ripeness but rather a characteristic of the variety.
Many people believe that the red color indicates sweetness, but it's not always the case. Color may be more intense in rhubarb grown in full sun than in part shade. Green varieties can be more productive -- and as sweet -- as red ones. If your plant's stems are all green, it's unlikely they will change color.