I don't wanna kill your enthusiasm but it's not really easy.
I mean, getting the colour of a pixel is piece of cake ( http://processing.org/reference/get_.html ), but you'll have to look into statistics algorithms. Quantitisation, differentiation, stuff like that.

But to get you started, do a for loop through all the pixels in your image and get their color wit get(). Compare every pixel with all the previous ones. If you have it in your list, increment its count. If not, add the colour to the list.

At the end, check the one with the highest count and you have the most frequent colour. When you get there, let us know.

Here's the code

Code:

int[][] colorList = new int[0][2]; //array that will contain all the colors in the image
PImage imageAnalyzed; //image to analyze
int colorToConvert,r,g,b;

void setup() {
  size(400,400);
  background(0);

  imageAnalyzed = loadImage("image01.gif");

  if (createPalettte()) {
    for (int k=0; k<colorList.length; k++ ) {
	println("Color: " + colorList[k][0] + "     Count: " +  colorList[k][1]); //prints the colors and the number of times it is prsent in the image (you'll notice that the values are not in rgb format)
    }

    //to convert it to RGB format you need to do this (and here I'll just do it for the first color)
    colorToConvert = colorList[0][0];

    r = colorToConvert >> 16 & 0xFF; //gets the RED
    g = colorToConvert >> 8 & 0xFF; //gets the GREEN
    b = colorToConvert & 0xFF; //gets the BLUE
    
    println("Color: " + colorList[0] + "  => r = " + r + "   g = " + g + "   b = " + b);
  }
}

void draw() {
}

//function which will populate the array
boolean createPalettte() {
  //loop through all the pixels of the image
  for (int i = 0; i < imageAnalyzed.pixels.length; i++) {

    boolean colorExists = false; //bollean variable that checks if the color already exists in the array
     
    
    //loop through the values in the array
    for (int j = 0; j < colorList.length; j++) {
	if (colorList[j][0] == imageAnalyzed.pixels[i]) {
	  int count = colorList[j][1];
	  colorList[j][1] = count +1;
	  
	  colorExists = true; //color already exists in the array
	}
    }

    //if the color hasn't been added to the array
    if (colorExists == false) {
	colorList = (int[][])append(colorList, new int[]{imageAnalyzed.pixels[i],1}); //add it
    }
  }
  return true;
}
 

It extracts all the different colors in a GIF image and keeps a count of them. The code should work for any image type, but like I said I really suggest using a gif in order to save time.

Cheers