Purple is not a good color...that is, for any human to be. It is the visual sign that the amount of oxygen in the blood is low, (technically speaking, the amount of oxygen bound to hemoglobin in the red blood cells is depleted). Blood testing can give an idea of the amount of oxygen actually in the blood, but the most immediate way to estimate this is by looking at the oxygen saturations that are seen on the pulse oximeter once the baby is being assessed. Usually, cyanosis correlates to oxygen saturations less than 80 to 85%.
It isn't uncommon for neonates to be blue or purple at birth, though often it is primarily the extremities that are most affected. When central cyanosis is present (head and trunk are also blue), it is more concerning in that it suggests a more significant degree of oxygen depletion. As you know, the greatest worry is that the brain (as well as the heart and kidneys, other organs very sensitive to lack of oxygen) may be hurt as a result of insufficient oxygen.
However, shortly after drawing those first breaths, most babies will pink up nicely. This is why APGARS are so important! A baby that is really "down" from lack of oxygen will take longer to breathe, may have a low heartbeat, may have poor tone and responsiveness, and may stay blue longer. A low one minute APGAR should prompt appropriate resuscitation, and most babies respond well. However, a low five minute APGAR is very alarming, and is strongly correlated to poor outcomes.
So, in your case, it would seem that your OB astutely observed the blue head/trunk, and promptly got your baby out where oxygen and resuscitation could be started. Given the risks of prolonged hypoxia, (look up hypoxic ischemic encephalopathy, HIE), my hunch is that it was the right thing to do. I'm assuming your baby's APGARS were fine and your little one is healthy and well?
Oh, and as to the cause of the hypoxia, it can be very difficult to determine and is sometimes multifactorial. Cord compression or occlusion (from a variety of reasons--compression in the birth canal, nuchal cord around the neck, knotted cord, clot in the cord, etc.), placental insufficiency, decreased blood flow due to uterine contractions, high maternal blood pressure are all possible reasons, though there are many others. I wouldn't assume that the same situation will happen during subsequent pregnancies, (assuming you don't have an underlying diagnosis that you know of that could have contributed, such as high blood pressure, a clotting disorder, etc.).
Anyway, I'm sorry the experience wasn't what you had hoped it would be. But, a big hug and congratulations on the best role of your life: being a mother!