Or you could go through the color challenge and find some you think could be done better and leave person a message to please contact you on Discord with your username then ask permission. ;-) But asking for submissions would be easier as you'd be sure to get people who wanted to learn that way and not someone who could be offended by the suggestion their composition could be improved. Also if the Photography channel gets more active, you could ask there if anyone had shots they were willing to submit for lessons and have done publicly.