A correct gas flame picture can be a good indicator of how your gas appliance is burning. A good flame will be sharp and blue, especially right next to the burner. Some yellow colouring can be ok at the top of the flame as this can be caused by different reasons. However, if you are noticing yellow/ orange at the bottom of the flame then get checked for Carbon Monoxide as the appliance could be emitting CO gases into the air.
Obviously, you cannot really totally on the flame picture and having a Carbon Monoxide Alarm installed is the most important decision you can make.