The reason they are blue is the 20,000k. The lower the k the witer the light to yellow around 6500k. basically the higher the k the more vibrant the color you will see (provided the water is well balnced) the lower the K the more growth you will see. there are a lot of factore that play into it though as far as color and growth. Thats just a basic difference. Also with 20k you may not need actinic suplimentation. matter of taste.