@TheGanjaMan, Spot on with the LED! The closer the better with a 100w LED as long as its not to hot.
Could you explain the type of yellowing you are seeing? Most cases of Yellowing this early are due to Nitrogen deficiency, Nutrient burn, or PH fluctuations. If It is excess nutrients causing the problems you can just flush the plants with plain PHed water for a few day before starting them back on a reduced nutrient schedule.
Here is a handy guide to help identify some problems that occur!
http://loudclouds.co/wp-content/uploads/2016/07/marijuana-deficiency-chart-jorge-cervantes.jpg