would the answer here be helpful to you?

  
      stackoverflow.com
  
  
      
    
  

  convert a grayscale image to a 3-channel image 222



  python, numpy



  asked by
  
  
    Dylan
  
  on 11:34PM - 18 Oct 16 UTC


  
  
    
    
  
  


They just make RGB channel all equal to the grayscale value. I suspect it should be good enough from deep learning perspective, though the actual image can be visually distorted a bit. BTW, if you use a pretrained model, I high recommend you fine tune it a bit. Cause I think it is likely the pretrained model is trained on real RGB image.