How to: Overlapping of gray dots colors in pygame

i am working on a pygame with random moving dots. The dots are moving across a black surface. The aim is to add upp the RGB-Colors of the dots in case they overlap. In the following image are 3 dots which do overlap. How can i add up the RGB of the Dots? Example in the Image are three dots overlapping. The part of a dot which is overlapping should have the color [3*250/N, 3*250/N., 3*250/N] with N = 4 the part should have the color [187.5,187.5,187.5]. The parts of the dots that are not overlapping shall stay the basic dot color [250/N, 250/N, 250/N].

I dont know how to add up the RGB colors. Any ideas?

And the current code:

import math
import random
import pygame
import random

N = 4

class focalspot(object):
    def __init__(self,dirnx=0,dirny=0,color=(250/N,250/N,250/N)):
        self.pos = 100,100
        self.dirnx = 0
        self.dirny = 0
        self.color = color


    def move(self, dirnx, dirny):
        self.dirnx = dirnx
        self.dirny = dirny
        self.pos = (self.pos[0] + self.dirnx, self.pos[1] + self.dirny)
        if self.pos[0] >= width:
            self.dirnx = -self.dirnx
        if self.pos[1] >= width:
            self.dirny = -self.dirny
        if self.pos[0] <= 0:
            self.dirnx = -self.dirnx
        if self.pos[1] <= 0:
            self.dirny = -self.dirny

    def draw(self, surface):
        pygame.draw.circle(surface, self.color, self.pos, 80, 0) 

dots = [focalspot() for i in range(N)]

def redrawWindow(surface,dots):
    global rows, width
    surface.fill((0,0,0))
    for dot in dots:
        focalspot.draw(dot,surface)
    pygame.display.update()


def main(flag,dots):
    global width, rows
    width = 400
    win = pygame.display.set_mode((width, width))

    counter = 0
    clock = pygame.time.Clock()    
    while flag:
        pygame.time.delay(50)
        clock.tick(10)
        for dot in dots:
            dot.dirnx = int(random.uniform(-10, 10))
            dot.dirny = int(random.uniform(-10, 10))
            focalspot.move(dot,dot.dirnx,dot.dirny)
        redrawWindow(win,dots)
        pygame.image.save(win,'temp'+str(counter)+'.png')
        counter =+ 1
        print(counter)



main(True,dots)

You should use the Surface class to draw your circles. This allows you to use the special blitting flags, e.g. BLEND_ADD, which will do what you want:

import pygame

class Dot:
    def __init__(self, pos=None):
        self.image = pygame.Surface((50, 50))
        self.image.set_colorkey((2, 4, 6))
        self.image.fill((2, 4, 6))
        pygame.draw.circle(self.image, (50, 50, 50), (25, 25), 25, 0)
        self.rect = self.image.get_rect(center=pos or (0, 0))

def main():
    screen = pygame.display.set_mode((400, 400))

    dot = Dot()
    dots = [dot, *[Dot(pos) for pos in ((50, 50), (100, 200), (120, 230), (200, 300), (300, 150))]]

    clock = pygame.time.Clock()    
    while True:
        for e in pygame.event.get():
            if e.type == pygame.QUIT:
                return

        dot.rect.center = pygame.mouse.get_pos()

        screen.fill(pygame.Color('black'))
        for d in dots:
            screen.blit(d.image, d.rect, special_flags=pygame.BLEND_ADD)
        pygame.display.flip()
        clock.tick(60)

if __name__ == '__main__':
    main()