Python code: how to darken randomised colours?

Hello and thank you for your ongoing help. I would like to make the randomised colors produced below using RGB values darker, is this possible and any help received would be much appreciated! Please note that I have commented out the heart rate inputs and simply set it at 50bmp. If you change the value, you should see the animation change colour and speed. Hope this makes sense! thanks again, Tina

##from pulsesensor import Pulsesensor
from Tkinter import *
import random
import time
import struct

##p = Pulsesensor()
##p.startAsyncBPM()

tk = Tk()
WIDTH=1280
HEIGHT=720
canvas = Canvas(tk, height=HEIGHT, width= WIDTH)
#canvas = Canvas(tk, bg="light gray", height=HEIGHT, width= WIDTH)
tk.title("drawing")
canvas.pack()

def getColor(speedCoef, green):
    prop = speedCoef - 0.2
    prop /= 2

    red = prop

    if red < 0:
        red = 0
    if red > 1:
        red = 1

    red *= 255
    blue = 255 - red


    #print("red: %d  green: %d  blue: %d" % (red, green, blue))

    rgb = (red, green, blue)

    response = "#"
    response += struct.pack('BBB', *rgb).encode('hex')

    return response


##below is the class to create multiple balls that are coloured
##and move and detect the edge and bounce

class Ball:
    def __init__(self, color, size):
        xPos = random.randrange(90, WIDTH-90)
        yPos = random.randrange(90, HEIGHT-90)
        self.size = size
        #stipple="gray25"
        self.shape = canvas.create_oval (xPos-size*0.5, yPos-size*0.5, xPos+size*0.5, yPos+size*0.5, fill=color, outline = '', stipple="gray75")
        self.xspeed =random.randrange(3,5)
        self.yspeed =random.randrange (3,5)
        self.green = random.randrange(10, 50)

    def update(self, speedCoef):

        color = getColor(speedCoef, self.green)

        canvas.itemconfig(self.shape, fill=color)

        canvas.move(self.shape, self.xspeed * speedCoef, self.yspeed * speedCoef)
        pos = canvas.coords(self.shape)

        middleX = (pos[0] + pos[2]) * 0.5
        middleY = (pos[1] + pos[3]) * 0.5
        newRadius = self.size * 0.5 * ((-(speedCoef-10) + 4) * 0.25)

        canvas.coords(self.shape, middleX-newRadius, middleY-newRadius, middleX+newRadius, middleY+newRadius)

        if middleY>=HEIGHT-30 or middleY<=30:
            self.yspeed=-self.yspeed
        if middleX >=WIDTH-30 or middleX <=30:                
            self.xspeed=-self.xspeed




colors=["#290404"]

##this is make 100 balls
balls=[]

for i in range (10):
    balls.append(Ball(random.choice(colors), random.randrange(150, 170)))



##index = 10
##this is to call the balls

while True:
##    bpm = p.BPM
    bpm = 150
    if bpm >0:
     print("BPM: %d" % bpm)
    speed = bpm * 0.01
    speed *= speed
## the following 2 numbers define the BPM range (0.2 = 45bpm, 2.2 = 150bpm)

    if speed < 0.2:
        speed = 0.2      
    if speed > 1.2:
        speed = 2.2
        #change bg color by taking speed (from bpm above) and updating the canvas
    bgcolor = getColor(speed, 25)
    canvas.configure(background=bgcolor)
    #print("speed: %f" % speed)

    for ball in balls:
        ball.update(speed)

    tk.update()
##    time.sleep(0.01)
tk.mainloop()
p.stopAsyncBPM()