You can use the loadpixels function to load each pixel into the pixels[] array which can then give you the colour infomation you need, for example this code will get the colour of the pixel at (20,200);

color px;

int x = 20;
int y = 200;
float redvalue = 0;

loadPixels();
px = pixels[(y*width)+x];
redvalue = red(px);

How if there is a lot of red color for example for an object, how to find its density and the center of it?

That is an expensive thing to do as far as I know. You probably will have to find every pixel that is red, store their (x, y) coordinates, and then average their coordinates to find the center (or whatever kind of centering technique you need).

What do you mean by expensive thing to do?

I mean that it requires a lot of calculation:

I did it:

Without having read the whole thread, I see a possible error in your code:

1. int y0 = avglocation/currentFrame.width;
2. int y = floor (y0);

floor() on an int is useless...
And see Why does 2 / 5 = 0 instead of 0.4?, as it might be the source of your issue, if the width is greater than the avgLocation (although the floor would lead the same result).

The reason why I put 2/5 = 0 instead of 0.4 because we should not have the pixel 0.4 in y coordinate. but we have 0 coordinate in y. It is my logic then, there is no decimal in going through every pixel of an image. What do you think?

That's OK if you are aware of the issue, lot of people overlook it.
I only commented on the last code snippet, that was lacking context.
Try to add a println() of x, y, perhaps they go out of the visible area.