I have just been putting water int he radiator (there is a leak) and ignoring the dex cool - so you put water in the radiator in the front of the engine and then put coolant in the other one (back left) where is's labeled dex cool???
Both the resevoir (Labeled Dex Cool) and the radiator are part of the cooling system. By adding water to the radiator, you are diluting the 50/50 ratio of coolant and water. Water has a higher freezing temperature, and lower boiling temperature than Dex Cool. It would be wise to fix the leak, then bring the system back to a 50/50 coolant water mix to help protect the engine during extreme hot or cold ambient temperatures.