Ok so the coloured part of the head on the coffee is diluted with the coffee itself, so has a higher water content than the rest of the head and thus a higher heat conductivity. The more pure microfoam you get in the head of the coffee, the slower you lose heat, so serving with latte art has the purpose of preventing the coffee from cooling too quickly. HOWEVER you should stir away the latte art, because of the differences is coffee concentration of the crema vs the microfoam, you'll get a strong first taste from the crema and the rest will seem weak.
