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What you are doing wrong seems quite obvious to me. However my python knowledge is limited so I will just explain it in plain text. It seems you are applying your mask function for both green and blue balls on the same input. This is dangerous, since the will result into a double operation on the same data.

What you should do in some sort of pseudo code - you will have to check if it works, haven't got a python implementation running for the moment -

import cv2
import numpy as np

cap = cv2.VideoCapture(0)

while(1):

    # Take each frame
    frame = cap.read()
    frame2 = frame.clone()
    frame3 = frame.clone()

    # Convert BGR to HSV
    hsv_blue = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
    hsv_green = hsv.clone() //this will make a copy of the HSV image

    # define range of blue color in HSV
    lower_blue = np.array([110,50,50])
    upper_blue = np.array([130,255,255])

    # define range of blue color in HSV
    lower_green = np.array([50, 100, 100])
    upper_green = np.array([70, 255, 255])

    # Threshold the HSV image to get only blue colors
    mask_blue = cv2.inRange(hsv_blue, lower_blue, upper_blue)
    mask_green = cv2.inRange(hsv_green, lower_green, upper_green)

    # Bitwise-AND mask and original image
    res = cv2.bitwise_and(frame, frame, mask_blue)
    res2 = cv2.bitwise_and(frame2, frame2, mask_green)

    cv2.imshow('frame_blue',frame)
    cv2.imshow('mask_blue',mask_blue)
    cv2.imshow('res_blue',res)

    cv2.imshow('frame_green',frame2)
    cv2.imshow('mask_green',mask_green)
    cv2.imshow('res_green',res2)

    k = cv2.waitKey(5) & 0xFF
    if k == 27:
        break

cv2.destroyAllWindows()

Check first if these both work seperately, then I will guide you to combining them.