You're on the deck catching rays, and even though you know you aren't supposed to tan without sunblock, you figure you'll be able to tell if you start to get really burned. Sure enough, after a little while you can see a redness coloring your skin, but it doesn't look too bad. Five or six hours later, the real burn begins appearing, and it's too late now to stop it.
What just happened is a kind of pump-fake from the system that runs temperature-balance in your body. People often assume it's possible to watch sunburn mounting on your skin, but this is a serious mistake. The red coloration that comes after an hour or so is not the first sign of tanning at all; that redness is caused by an increased flow of blood toward the skin due to excess body heat.
It's the same effect that gives people a ruddy glow after exercise: the over-heated body attempts to readjust its temperature by directing more blood toward its surfaces, which has the effect of radiating away heat. This "flush" will disappear as soon as you cool down.
Therefore, it's no indication as to whether the skin itself is becoming burned. The redness of a burn looks the same because it's also caused in part by increased blood flow-but with an actual burn, the blood is being shunted outward much more rapidly in an effort to repair already damaged cells. By the time the blood starts being sent to the aid of your traumatized skin, several hours have elapsed since that deceptive "first blush" has faded. It's only then that you can start to feel how much or how little of a burn you actually received.