In the passage in which the shadows "on Ralph's face are reversed with green above, bright below from the lagoon," they are as they were when he stands on his head earlier in the chapter. Thus, there is here an indication that things are reversed as
Ralph looked through him [Piggy]. Here at last was the imagined but never fully realized place leaping into real life.
Ralph smiles as he realizes that he is in an inverted world in which the children are in charge. This is a world which he has dreamed about--the reality catches him. And, of course, the significance of this moment cannot be missed by the reader who, in retrospect, understands the foreshadowing of problems as the boys are on their own without the assistance and guidance of civilization, as represented by adults. With Ralph it is a new beginning (green) born from the sea (blue).
I'm a little unsure what you're referring to here, but I don't think it's scratches. Piggy's the one who got all scratched up as he climbed through the brambles and thorns; Ralph appears to be immune to such mundane things, at least in the beginning.
I'm guessing you mean this passage: "All the shadows on Ralph's face were reversed; green above, bright below from the lagoon. A blur of sunlight was crawling across his hair." If so, it's a fairly simple observation. This is a relatively allegorical novel, meaning the characters and events are relatively symbolic. Ralph is the character who most represents physicality--he's an athlete, and when he gets excited he stands on his head. He has difficulty collecting his thoughts, but he can swim and dive quite well and is envied for his abilities by the least physical character, Piggy.
Now back to his face. Notice the essential elements of earth (green), water and sky (with fire soon to come, of course). These are the physical components which represent Ralph.
i think that it was the scratches from walking around the forest too much. and it would be important because it foreshadows that in the future, ralph and others would be in harm. and light basically stands for harmony and peace and kindness and also innocence and dark stands for evilness, nightime and the horros. and i'm not sure about the last one. we're also reading LOTF.