The reason cooked rhubarb turns brown mostly has to do with the variety and the way it's cooked. The Strawberry rhubarb variety will look pinker when cooked but it's not a common variety and harder to find. Roasting rhubarb can also help rhubarb preserve a pinker color. So if you are looking for the pink color, I recommend you roast it. You can also add a little red food coloring. Your cookware can also affect the color of your cooked rhubarb. Because rhubarb is acidic, if you cook it in a metal pan like copper, aluminum, the rhubarb will turn brown. But in most cases, it's simply the variety and stove top cooking method that causes it to turn a brownish color.