There are many reasons why this may occur. It may be due to changes in cementitious materials, fine aggregate sources in subsequent batches, or different loads of concrete. Sections of your concrete may have been poured with different water-to-cement ratios, meaning some loads of concrete were poured with more added water than others. All concrete loads needed to complete your project should all be batched with the same slump.

Another reason for differing colors could be attributed to calcium chloride being used in the pour, assisting it to set faster. This additive is most commonly used in cold weather conditions - such as in the fall and spring. Setting accelerants should always be used in lesser amounts if possibly.