The difference is the kelvin rating that the previous guy refered to. The lower the Kelvin(K) the yellower the color. The higher the K the more blue you will see in the light. Most people prefer either 1)10,000K bulbs or 2)the brighter 6500K bulbs with actintic pc, vho, or flouresence supplements. There are bulbs that run all the way to 20,000K which are very blue. Most people it seems fall into the middle of the road 10000k or 6500k. The big discriminator for some is the price difference. For example a 250w 4200k bulbs is usually about $10 from what I have found. The 6500k bulbs are around $59-$89. The 10,000k run about $90- $120. I am not sure if there is a 20,000k bulb for 250w. I hope that I have helped and not caused to many more questions.
RLTW