Question about tank lighting!

Check out the Kelvin ratings on the bulbs you see at the store, which will give you an idea of how 'blue' the bulb will appear; keep in mind, everyone sees light slightly differently. Higher numbers indicate a bluer colour - a 5000K bulb will look slightly yellow (like sunlight), 7600K may look 'crisp'; more white than yellow. A 10000K bulb is blueish white. It's possible you saw an actinic bulb - it's a BLUE bulb, but not full/broad spectrum, that (AFAIK) provides light specifically in a particular range (somewhere between 440-470nm I believe). It's incredibly blue, and I've read that it really makes the colours of some marine fish and corals 'pop'. I wouldn't want to light a FW tank with just actinic because it's not full-spectrum and may not meet the needs of the fish.