Uneven curing of the surface will result in uneven color. When using plastic sheeting over concrete use care to either entirely “tent” the surface or entirely lay the plastic sheet directly and evenly over the surface. Both efforts are extremely difficult, time consuming, and costly. There must not be any occasional “tents, bubbles or ripples”. If this occurs, a “pronounced mottled” (color from light to dark, etc.) appearance will result. What happens is that on warm days, each ripple becomes a miniature greenhouse where the evaporation-condensation cycle is repeated several times. The buildup of condensate water around the ripple and in low spots causes the variation in curing and therefore variations in color. This is especially true in concrete containing calcium chloride.
Air cured concrete (which is to say… lack of curing) exhibits the greatest amount of discoloration. Thoroughly moist-cured concrete exhibits the least. Uneven curing will result in uneven coloring. In my opinion, the final color of the surface of the concrete slab is greatly dependent on 1) the moisture content (water-cement ratio) at the surface during the time of the final finishing effort, 2) the intensity of that effort, and 3) the curing conditions.
Lastly, avoid changes in raw materials in the concrete, such as cementitious material or sand sources. These individual materials may differ in color. Stay with the same concrete supplier and the same mix design. Avoid heavy or varying dosages of calcium chloride and varying dosage rates or the intermittent use of high range water reducers (superplasticizers) that alter the water-cement ratio. Minimize inconsistencies.
Listed below are available publications that go into great detail about the subject.