Most folks' first thought is "Gaaa! I'm not using
Auto Color! I'm
calibrated. I post-process with Photoshop."
EDIT: You can now get a GREAT DEAL if you want your own version of i2e :)
The truth is nearly all of our customer returns on prints are when True Color has been chosen. We experience near zero returns when Auto Color is chosen. Why?
SmugMug's Auto Color is not what you're used to, the "one-button" fixes in applications like Photoshop, Paintshop, Picasa, and more. We have a
great help section that explains all about it. So why this piece too? Probably, it hits close to home for me, because as
SmugMug's House Pro, I see the True Color Returns every day.
Our Auto color will adjust your photos for exposure, color, and will also sharpen them just right, for print.
Some of the most common reasons for returns are when prints are
too dark, or
too red. Here are a couple of extreme examples, to show you the power behind
SmugMug's Auto Color.
Notice in this first example, the shot is 2 stops underexposed. This would look horrible in print, prints are much darker than what you see on screen, as prints use reflected light, while the image you see on screen uses backlit light. Notice how our Auto Color brings even this very underexposed image into a great exposure, one that will print very well.

And here's an extreme example of "too-red," it was shot in daylight, underneath a Red Japanese Maple Tree. Note how the skin tones in the image on your left, the magenta values are much higher than the yellow values. Look at the image on the right, after
SmugMug's Auto Color, even in this extreme example, the
skin tones are now much much better. Magenta is not higher than Yellow.

OK, so what do you do about this? Most folks should use (and default the gallery settings to) Auto Color if you don't do any processing to your images. When should you use True Color? Best is after you've checked in with
SmugMug's
House Pro (me, Andy) asking for a review of some sample shots.
If you can't see major differences in these examples, no worries - we'll help you
calibrate!
The toughest issues with digital are exposure and skin tones.
Exposure is an issue for two reasons. One is you and your customers
view the proofs on computer monitors, which light photos from behind,
and which are getting brighter every year. Prints are usually viewed in
the subdued lighting of homes, depending on reflected light, not the
backlighting that computer monitors use to make them so bright and
vibrant. The monitors fool you and your customers into thinking a dark
image will look good in print.
Here is a help section about that.
The second reason is digital cameras do not have the exposure latitude
of color negative film, far from it. So you have to be on your game to
avoid blowing highlights (like cheeks or foreheads) or having shadows go
black with no visible detail.
Exposure and latitude are such big issues with digital that the two
biggest things our autocolor option in the shopping cart does is (a)
brighten the photo and (b) bring the shadows up in exposure even more to
make the overall look more like film. It's telling that we get 90%
fewer returns when the autocolor option is chosen, even with pros who
think they adjusted their photos well in Photoshop.
Skin tones: when we get returns, if the issue isn't that the print is
too dark, it's that the consumer doesn't like the skin tones. Once
again, two issues:
1. With film, you either bought it adjusted for the light you're
shooting under, or you put a filter on your lens. With digital, you set
the white balance. If the white balance is off, so is the skin. You
mentioned shooting a color checker, which was often done with film.
With digital, it's more common to shoot a white card to set the white
balance. Your camera comes with preset white balance options, one for
sunlight, cloudy, shady, flash, tungsten light, fluorescent, etc. Leave
the setting on tungsten and take a portrait with flash, and you've got a
skin tone problem.
2. Flashes emit near-infrared light, which is red. Cheaper flashes
emit more. Never a problem with film, it's not sensitive to infrared.
Digital cameras are and fair-skinned caucasians reflect a lot of it,
making them redder than they really are, especially their zits. :-(
Bad news for graduating seniors...
The third thing our autocolor does is make skin tones look more like
they did with film, by identifying regions of skin and removing some
magenta and adding some yellow.
Unfortunately, almost all you read online and in Photoshop books is
about producing good landscapes, a very different thing than producing
good portraits. Landscapes are usually sharp and vibrant and were shot
with different film than portraits, which are usually soft and with less
contrast, at least where the skin is.
You'll read about color correcting landscapes by looking for a white
point and black point and making sure Photoshop knows they're white and
black. It usually works for landscapes but rarely for portraits that
may not have any white or black areas.
Here's a tutorial on skin tone that lets you open any shot in a program
like Photoshop and know whether you're in the ballpark with skin tones.
SmugMug Pros should remember too, that you can use the
Proof and Retouch feature, to see the color choice chosen by your customers, too, and you can change it, as needed.