When discussing the question of symbol, you need to know that these are a highly interpretive literary element and everyone can see them a little bit differently. To me, these symbols mean the following:
NAKEDNESS: This refers to the boys total wreckless abandon of the society in which they used to partake. They have completely discarded its rules and order.
PALM TREES: In my opinion, the palm trees stand with significant shadow over the boys which demonstrates the great length to which they would have to go in order to free themselves from the island. These trees are insurmountable to climb, they have nothing to grip and they pose great danger. The case for rescue is a similar obstacle.
COLORS: When the boys begin to paint their faces, we start to see more color emerge for specific reason in the text. The red in particular signifies danger and harm. This comes to fruition when Jack's crew begins to ultimately hurt the rest of those who refuse to convert to Jack's tribe.
LAGOON: This area is a place of safety for the boys. Their scheduled meetings take place close to here and they use the lagoon to bathe. It is a symbolic cleansing of the evil that wears more on them everyday. As Jack and his crew of evil move further into the forest, they move further away from the opportunity to experience the safety of this cleansing location. If you recall, when Ralph and Piggy were first alone on the island, these were moments of pleasure in the lagoon.