Dr. Greene's Answer
Babies’ blood vessels start off highly reactive and unstable. A mild change in temperature or position or mood can cause swift changes in the diameters of the blood vessels, with resultant color changes of the overlying skin. The most extraordinary example of this is the harlequin effect.
A sharp line from the forehead to the pubis divides the body into 2 vertical halves. One side turns dark red, the other quite pale. The overall effect is reminiscent of the bold patches of color on a harlequin costume.
This rare but dramatic event only occurs in the immediate newborn period. It usually begins when the baby is positioned on her side. The upper half of the body becomes pale, and the lower half deep red. Changing her position can reverse the pattern. If she moves a lot, the muscle activity will erase the color changes (rather like shaking an Etch-a-sketch toy).
The harlequin color change is most common in low birthweight infants, but can occur in any child. Babies who experience this once will often take on the harlequin pattern multiple times.
Still, the condition is as temporary as being a newborn. The harlequin color change is entirely harmless. It has never been associated with any permanent problem.