In line 8 you have declared and created an array of floats called r with

float[] r = new float[maxCount]; //radius

because it has been declared outside any of the methods (e.g. draw and setup) it is called a global variable can be used from any part of your sketch code.

Now inside the draw method (line 35) you have the statement
float r = red(img.pixels[loc]);

this will create a local variable called r of type float. Since it is declared inside a method it is called a local variable. Local variables are only available inside the method they were declared.

So by the time the program gets to line 43

if (d >= (tr + r[i])) insection = false; ////////////// there seems to be the problem
   
there are two variables called r, the global variable (array of floats) and the local variable (single float) and the Java compiler will pick the local variable in preference to the global variable.

The compiler will then see you attempting to use a float variable as an array

The solution I suggested was to rename the float variable on line 35 to something else so that the compiler will use the global variable i.e. the array of floats.

Looking at your code I can't see why the colour of the ellipses should change since although you grab the colours of each pixel with
       float r = red(img.pixels[loc]);
     float g = green(img.pixels[loc]);
       float b = blue(img.pixels[loc]);

you don't seem to have any code that uses these values !!!

To avoid this problem in future I suggest you declare all variables with unique names. If you want to know more about this problem you might want to investigate scope of variables


In reality the explanation is more complex than that because Java is an object orientated language but this should be enough to explain the problem in your code.