Concrete Color Variations
Concrete and its inherent parts are all natural products. Therefore, the color of the finished concrete is subject to a number of variables. Variables that can affect color are brands of cement, aggregates, temperature, humidity, finishing techniques, and curing practices. For these reasons, there is no warranty on the consistency of the color of the concrete.
Concrete needs warm temperatures in order set properly. In cold weather, contractors must use chemicals to accelerate the set times before the concrete cools. These chemicals usually darken the color of the concrete.
Reference Concrete In Practice #23 – “Discoloration”, for more detailed information regarding the color of concrete