First off you would want IC this rating allows for the fixture to be in contact with insulation. I have 20 of these installed in different rooms and used for different functions, they are all 6". The difference is the bezels or trim rings and the bulb type (sunlight or softwhite , spot or flood)

The temperature is usually a characteristic of the lamp (bulb) rather than the fixture. Are you talking about LED lamps? LED lights are great but very expensive.

6" recessed fixtures were popular in the 60's and 5" fixtures were popular in the 80"s. Both look very dated if you care about that. Today the most popular ones are 4" because the lamps are more hidden in the fixtures and far less visible.

Since the purpose of recessed fixtures is to lower your awareness of the light source and to be focus light on certain specific areas (instead of general illumination) I usually use Lightolier LyteCaster 3 3/4" fixtures.

The fixtures from Home Depot and Lowes tend to have plastic trim rings that turn brown with age and break if accidentally hit but they are cheaper than many other fixtures. I also avoid them because the cheaper brands often limit the wattage of the lamps in fixtures that must be used in insulated ceilings below a roof (air-seal, insulation contact). Also the lamps sold in these stores are usually of low quality or have been damaged so they don't last very long. Keep the receipt.

You might want to do some research on the lighting forum. There are some real experts on LED/cans over there.

The human eye is not capable of distinguishing the difference between 2700 & 3000degK

The Kelvin luminance scale was originated for the photography industry to indicated the white balance of photographic film.

Try a personal experiment..On a bright sunny day look out your window at a white object then look at a white object in the room under incandescent lighting. You will notice that the incandescent light emits a slight yellowish cast.

Now if you come back at night when its dark that same white object under the incandecent light will appear white and you won't notice the yellowish color of the light. That is because you eye has automatically compensated for the color balance of the light source.

In the photography business they were sometimes shooting outdoors under natural daylight, while other times they were shooting in a studio with incandescent lights, but if they used the outdoor film indoors the photos all came out with a yellowish cast.

It would be easy to make film dyes that would reproduce white under nearly any lighting condition, but they needed a standard so the film manufacturer could inform the photographer what type of film it was.

To create a standard they put a piece of pure carbon in a lab furnace and began heating it. It started out coal black, but as it was heated it began to glow, first a dull yellow, then a yellowish red, to a bright red to a reddish blue, then to a blue like the gas flame on a stove, and on until it was a pure white matching daylight white.

They recorded the temperatures as the colors changed and they determined that the glow from the carbon matched incandescent light at 3200 to 3400 degrees Kelvin and the bluish white light of daylight was at 5500 to 6000 deg kelvin.

Although there are dozens of special films for photography, basically the two that most consumers are familiar with are indoor film (32K) and outdoor film (64K).

If you have a digital camera it probably has a White balance control. Try an experiment for yourself. Set your cameras white balance to incandescent and shoot a couple pictures indoors, then switch flourescent and shoot the same pictures again, then switch to daylight and once again shoot the same pictures and you will see the difference in color balance.

Up until the late 90's primarily all we used for indoor lighting was incandescent so there was not much emphasis of telling you the color temp of the lights. Some incandescent and flourescent lights have an internal coating on the glass envelope and the lamps were listed as daylight balanced, but nobody paid much attention to that.

With the advent of the mini flourescents and LED lights we are seeing some rather radical color shifts so now the industry is labelling the lamps with the Kelvin Luminance number....Basically the hotter the Kelvin number the whiter the light output will be.