Hi Steve
After reading your question I have looked at a number of cement and concrete resources to find out more about concrete constituents and the processes involved in mixing and laying it.
I can't find a definitive answer to your question... but this is what I've learned -
The water : cement ration of the concrete can affect its 'cured' color... if there is a variation in this throughout a poured slab, there is likely to be a difference in the color. A high water : cement ratio will usually result in a lighter colored concrete and a lower water : cement ratio will usually result in a darker colored concrete.
In my understanding related to your question, and I have noticed it in expansion joints in sidewalks, is that perhaps when each slab is poured and cures the water may pool slightly more towards the center and away from the edges - leaving a lower ratio at the edges ----- thus darker edges.
As far as I understand there could be any number of reasons for the cracks and discoloration: preparation of area to poured, constituents of concrete/cement, hydration process, environmental conditions, contaminants....
I know that oxalic acid can be used to 'brighten' concrete... but if the actual water to cement ratio is the cause of discoloration - I don't imagine there is much you can do to change its color, short of staining/painting!!
check these links out...
www.cement.org
http://www.lymanrichey.com/readymixed/FAQs.asp
http://www.nrmca.org/
http://www.prmconcrete.com/solutions.htm
http://www.concretenetwork.com/
Hope this helps a bit buddy
