why yellow-green early?

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Early spring leaves are so beautiful.

But, I need help.
 
I’m having trouble remembering this one. I’ve put so much stuff in my brain, I think some of it is oozing out of my ears. I tend to wear my hair long to hide the leakage.
 
Why are new spring leaves yellow-green? But by summer, these same leaves have turned to a darker blue-green?
 
As I recall it has something to do with spring leaves having more cholorophyll A, which is a yellow-green pigment. Later on, the leaves produce more cholorophyll B, which is a blue-green pigment.
 
But, if this is true, why? Is there a practical reason that cholorophyll A emerges first?  

I even dug out my botany textbook from college but couldn't find an answer.
 
Is there a botanist out there?
 
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