I assume you have a 4x65w PC fixture.The K ratings on bulbs describe there color temperature.This is true for the most part except for the 7100k actinic.I dont know why they list that bulb with that kelvin rating.Here's the run down;6500k(actually 6.5k) is yellowish,it is most similiar to the sun on a clear day at noon.10,000K(called 10k)is what the industry dubs a pure white bulb.Now for the actinincs,actinic bulbs are usually availiable in 2 kinds.They are determined by there spectral analysis peaks.A true actininc,often reffered to as an act03 has a spectral peak @420nm.The 7100k(dont ask how they came up with that #)has a spectral peak between 450nm and some will peak closer to 470nm.Now to understand what all this stuff means,you must first undderstand the colors that make up the spectral plot and the peaks in these plots.Basically,anything below 400nanometers=nm is ultra-violet radiation.From 400 to about 435 is violet/purple.435 to about 480 is blue.For a better understanding of spectral plots and the colors they represent i'll refer you to this thread