docstr: extract average value of green color from ROIs
code: def extract_color(self, ROIs):
      """
      extract average value of green color from ROIs
      """
  
      # r = np.mean(ROI[:,:,0])
      g = []
      for ROI in ROIs:
          g.append(np.mean(ROI[:, :, 1]))
      # b = np.mean(ROI[:,:,2])
      # return r, g, b
      output_val = np.mean(g)
      return output_val
  
file_path: heart-rate-measurement-camera/new_update/signal_processing.py
imports: import cv2
  import numpy
  import time
  from scipy import signal
label_desc: Use image node_id 2 for calling the Signal_processing obj's underlying member method code with example usage: obj.extract_color(ROIs) and returns: output_val