In the distant past light bulbs came in only 1 color, incandescent. Now days there are some bulbs coming in a variety of color temperatures and others that don't even bother to list on the package what it is. This is the main reason that compact florescent bulbs took so long to catch on in the US as people hated the color of the things. Now it's possible to get bulbs that you like if you do a little experimentation. I would not buy a case of CFL or LED bulbs without buying 1 first to see what you like. Specific color temperatures are measure in degrees kelvin, which i believe is the color of light as radiated from a black body at that temperature or something like that. The lower end of the numbers start around 2700k which is considered "warm" white. The "bright" white that you'll see are generally in the 3000k to 3500k range. And then the "daylight" white which can be as high as 6700k
"Warm" whites.
A 60 watt regular bulb looks to me right at the 2700k mark. I have seen some CFL bulbs and some LED bulbs do this as low as 2000k and I think those are useless too. Many of the 2700k bulbs used to look more green than white. However, if you have just a few lamps you want warmer colors. In my experience with lower light levels you want them to be warmer. A single daylight bulb makes a room look like an overcast day where a single warm white bulb just provides regular light as we're used to. Most lamps that are unlabeled as to their specific temperature are warm whites. I would not go below 2700k if possible as they start to look orange or green depending on the phosphors used.
"Bright" whites.
My favorite CFL bulb is the 110 watt equivalent from Sylvania who's color temperature is 3500k. I have an array of these in the cans in the kitchen and because there are 8 of them they provide wonderful working light in there when they are on. The single lamp in the corner of the living room that provides just walk through light I would never use one in because the color is too higher. The higher the color temp the more light you need to make is useful.
In between bright and daylight there is a wide range of other offerings though you may not find them at your corner grocery. I have lots of C8 tubes in the garage at 5000k and a few whiter CFL's in other places that I like a lot. But again a single one would look like a cloudy day. If you need a lot of good worklight and don't like the blue industrial feeling of the daylight lamps then these bulbs are for you.
"Daylight" whites.
Personally I have no use for anything above 5000k, I just don't like it. Daylight white bulbs are generally in the range of 6000k to 6500k and range up to blue, bluer and horrid in my opinion. If you need a lot of worklight than an array of these might be to your liking. I would buy just one to see what you think of it before investing though. I know that people in other countries often tend to really like the cooler temperatures (even though they have higher temperatures, they are cooler ;) but I guess I am a victim of some kind of American indoctrination as I don't.
"Full Spectrum"
I understand what these are supposed to be... Looking at the actual spectrum of light emitted from any phosphor you'll find peaks and valleys. They dont emit in a straight line so some colors will be accentuated and others will be muted. A full spectrum bulb has specially made phosphors to try to emit more of a straight line across it's spectrum. They tend to measure higher in the color temperature too. I've experimented a bit with them and just dont see the advantage for the much higher cost. They will render colors better if you're trying to discern the differences between adjacent entries in the pantone color sheet, but for anything else I do not personally see any reason to use them over an array of 3500k bulbs which are wonderful to work under.
"CRI"
Many lamps will advertise their "CRI" or color rendering index rather than their color temperature. But this isn't really as useful as you wont know what the color of the lamp is, only how well it is at reproducing other colors. Again, as in the full spectrum bulbs, unless you're doing color reproduction where it matters, a high CRI is not necessary. A really low CRI though would be bad though.
Notes on LED bulb offerings:
LED's compound the problem again. A white LED is really a blue LED with a phosphor spread on top. Cheap or older white LED's are very blue because of this. Modern ones are solving this problem with better phosphor application and better phosphors but they still tend towards the daylight or worse unless specifically marked as warm. (and even then, as in those warm white floods from LOA I spoke of in a previous entry, they may be completely mislabeled) I've bought warm white LEd's that are orange, I've bought warm white led's that are pinkish, I've bought warm white led's that are greenish. The warms for LED's tend to be very warm, like a bulb dimmed to just glowing. The problem is they aren't standardized. The higher power LED's out there aren't even made knowing exactly what they are going to output when they are made, they are binned after production based on measuring the output. And the binning process adds to the cost. So most folks will just grab a handful and solder them in giving a wide range even within the same array. Just like in CFL bulbs you get what you pay for. Cheap LED lights will not last and will output blue light but only for a short time as they dim down to nightlight status. And while really good expensive LED lamps are out there, they are very expensive...
(letters from the future: looking back on this entry from 2012 I can only add that really good warm white LEd bulbs are now available for prices that wont put you in the cardiac ward. My personal favorites are the slightly odd looking yellow phosphor ones from Phillips that Home Depot carries. They are excellent, they output lots of light, they are indistinguishable color wise from an old fashion light bulb and they dim all the way to nothing smoothly. The 75 watt equivalent ones are still around $40, but for $25 you can get the 60 watt equivalent and $21 the 40. Get one and use it when some regular bulb blows, you wont be unhappy with it!)
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