Rhubarb varies quite a bit in color. Some stalks are deep vibrant red, and others vary from green to pink. The flavor will be the same, but the color of your butter may vary. Often supermarket rhubarb is bred for a bright color, and I always try to make sure I get nice bright stalks. My original batch of rhubarb curd, back in 2013, below, was much paler, but no less delicious.