Those 4-foot bulbs come in four colors/ temperatures.
Some people say “warmer” is better, but isn’t that more yellow than natual light?
And the colder bulbs seem to give off more lumens. That’s more efficient, right?
What color is natural light?
I assume by “natural light” you mean sunlight.
There’s color temperature (sunlight is around 5200k, give or take) and spectrum levels. Sunlight has a fairly even spread of color, with a slight peak around the green/yellow area. Normal bulbs don’t. There are manufacturers that make bulbs that match the spectrum levels of sunlight. (Like here or here) Or search for “full spectrum fluorescent”
You get more lumens for the buck with mercury vapor lamps than with fluorescents or incandescents, but the frquency is wrong for the human eye. This chart of the photoresponse of plants, and the human eye, can be helpful in picking the right bulb for any purpose:
It depends on how you define “better.” Most people find warmer light (redder) more pleasant for household lighting than cooler (bluer), or full-spectrum (sunlight) bulbs. We get this sort of warmer light from household incandescent bulbs - so to match this you’ll want a warmer light.
Be careful, though - “warmer” and “cooler” are the exact opposite of color temperature. Color temperature can be thought of as “the color of light emitted when a filament is heated to a certain temperature.” So a filament that’s just starting to glow will be red-hot, and as it gets hotter it will be white-hot, and as it gets even hotter it will move even further into the blue range. So a hotter filament emits a “cooler” light.
For household use, you probably want a color temperature no higher than 27K.
This should be 2700K, of course. I was using a shorthand used by the place where I buy bulbs.
I see the chart, but I can’t compute. Not sure how mm relates to K
Here are the choices I see at the Home Depot
3000K - 3200 lumens
3500K - 3200 lumens
4100K - 2200 lumens
5000K -2200 lumens
6500K - 2325 lumens
All of these seem more natural than 2700K
And no, I don’t desire yellow light just because someone else thinks it’s cosy, I want natural light.
But I guess the K doesn’t specify what color it will look like?
Why do you come to that conclusion? Color temperature (K) should directly correlate to color appearance, otherwise I’m not sure what it’s measuring or indicating.
I would think bulbs should be rated by 4 colors.
Doesn’t the eye have four different chemical dyes that respond to light differnt ways?
Red, Green, blue in the color cones and visual purple in the B&W rods.
So K is misleading, since it is an average, where red+blue averages to green, but the green receptor may see very little. While a green light with the same K would excite the green but not the red and blue.
Aren’t fluorescents actually mercury vapor lamps with a fluorescent (funny that 
) coating which absorbs UV and re-emits as visible light?
You sound as if you think there is one, single ‘natural’ light. There isn’t.
Sunlight itself varies over the day, from the “rosy dawn” to noontime to the glare of mid-afternoon to the fading lavender tones of early evening.
And of course, your location on the earth & the season affect this greatly, too. A mid-winter afternoon here in Minnesota vs. the same day in Florida will have a very different ‘color’ of sunlight.
The color temp given for natural sunlight is just an average, for something that varies every day, all year long.
Yup, you lose some efficiency, but get a better spread of wavelengths with a coating.
You can’t directly compare the lumen ratings with respect to energy consumption. Lumens are weighted by the wavelength sensitivity of the human eye, meaning a more reddish light may actually use more energy but look dimmer.
There are CIE standard illuminants with defined spectral power distribution, for example D55 (morning), D75 (overcast daylight), and the most widely used standard for daylight is D65 (noon daylight). (The numbers correspond to the equivalent color temperature.) Specially calibrated bulbs are available (printers use them in color proofing booths for example), but I think you definitely wouldn’t want having to buy those for home use. So if you’re not concerned about the delta-E’s of your color reproduction and just want a light that looks as white as daylight, buy the 6500 K bulb.