In Chapter 6, Lord of the Flies, the sentence "Jack went red" (right after Ralph told Jack "You're a hunter") implies that Jack is:
a. Angry at the boys for hesitating
b. Flushed with the exertion of the climb
c. Embarrassed to have his fear exposed
d. Fearful that perhaps he must go hunting
e. Sunburned from the long day in the sun
If Jack was sunburned, he would have gradually acquired a red complexion. The same goes for being flush from the climb. To say that Jack "went red" indicates that Jack became angry or embarrassed. His face could have turned red but the notion of "going red" or "seeing red" often implies a dramatic change to anger or embarrassment. It is pretty clear, from the context, that all the boys were afraid of the beast. Although Jack is usually impatient with the others, he is not angry that the boys hesitate in this context. Earlier in this chapter, they decided to explore the other part of the island to see if the beast is there. When they approach the rock cliff (castle), fear sets in and this is indicated when the usually noisy band of hunters becomes silent:
Behind Ralph the tall grass had filled with silent hunters. Ralph looked at Jack.
"You're a hunter."
Jack went red.
"All right. I know."
Ralph was implying that since they were hunting for the beast, Jack, the leader of the hunters, should go in first. Jack is probably scared at going in first but he's more embarrassed at having this fear exposed in front of the others. Ralph lets him off the hook by volunteering to go in first himself. After Ralph ventures forward, Jack eventually summons the courage to join him. Jack says, "Couldn't let you do it on your own." Jack isn't there for moral support; he's there so he can look as courageous as Ralph.